In front of the primitive streak two longitudinal ridges, caused by a folding up of the ectoderm, make their appearance, one on either side of the middle line (Fig. 16). These are named the neural folds; they commence some little distance behind the anterior end of the embryonic disk, w^here they are continuous with each other, and from there gradually extend backward, one on either side of the
anterior end of the primitive streak. Between these folds is a shallow median groove, the neural groove (Figs. 10, 17) . The groove gradually deepens as the neural folds become elevated, and ultimately the folds meet and coalesce in the middle line and convert the groove into a closed tube, the neural tube or canal (Fig. 18), the ectodermal wall of which forms the rudiment of the nervous system. After the
coalescence of the neural folds over the anterior end of the primitive streak, the blastopore no longer opens on the surface but into the closed canal of the neural tube, and thus a transitory communication, the neurenteric canal, is established between the neural tube and the primitive digestive tube. The coalescence of the neural folds occurs first in the region of the hind-brain, and from there extends
forward and backward; toward the end of the third week the front opening (anterior neuropore) of the tube finally closes at the anterior end of the future brain, and forms a recess which is in contact, for a time, with the overlying ectoderm; the hinder part of the neural groove presents for a time a rhomboidal shape, and to this expanded portion the term sinus rhomboidalis has been applied (Fig. 18). Before the neural groove is closed a ridge of ectodermal cells appears along the prominent margin of each neural fold ; this is termed the neural crest or ganglion ridge, and from it the spinal and cranial nerve ganglia and the ganglia of the sympathetic nervous system are developed. By the upward growth of the mesoderm the neural tube is ultimately separated from the overlying ectoderm.
The cephalic end of the neural groove exhibits several dilatations, which, when the tube is closed, assume the form of three vesicles; these constitute the three primary cerebral vesicles, and correspond respectively to the future fore-brain (prosenceyhalon) , mid-brain (mesencephalon), and hind-brain (rhombencephalon) (Fig. 18). The walls of the vesicles are developed into the nervous tissue and neuroglia of the brain, and their cavities are modified to form its ventricles. The remainder
of the tube forms the medulla spinalis or spinal cord; from its ectodermal wall the nervous and neuroglial elements of the medulla spinalis are developed while the cavity persists as the central canal.
The notochord (Fig. 19) consists of a rod of cells situated on the ventral aspect of the neural tube; it constitutes the foundation of the axial skeleton, since around it the segments of the vertebral column are formed. Its appearance synchronizes with that of the neural tube. On the ventral aspect of the neural groove an axial thickening of the entoderm takes place; this thickening assumes the appearance
of a furrow — the chordal furrow^ — the margins of which come into contact, and so convert it into a solid rod of cells — the notochord — which is then separated from the entoderm. It extends throughout the entire length of the future vertebral
column, and reaches as far as the anterior end of the mid-brain, where it ends in a hook-like extremity in the region of the future dorsum sellse of the sphenoid bone. It lies at first between the neural tube and the entoderm of the yolk-sac, but soon becomes separated from them by the mesoderm, which grows medial- ward and surrounds it. From the mesoderm surrounding the neural tube and notochord,
the skull and vertebral column, and the membranes of the brain and medulla spinalis are developed.
THE PRIMITIVE SEGMENTS.
Toward the end of the second week transverse segmentation of the paraxial mesoderm begins, and it is converted into a series of well-defined, more or less cubical masses, the primitive segments (Figs. IS,
19, 20), which occupy the entire length of the trunk on either side of the middle line from the occipital region of the head. Each segment contains a central cavity — myocoel — which, however, is soon filled with angular and spindle-shaped cells.
The primitive segments lie immediately under the ectoderm on the lateral aspect of the neural tube and notochord, and are connected to the lateral mesoderm by the intermediate cell-mass. Those of the trunk may be arranged in the following groups, viz.: cervical 8, thoracic 12, lumbar 5, sacral 5, and coccygeal from 5 to 8. Those of the occipital region of the head are usually described as being four in number. In mammals primitive segments of the head can be recognized only in the occipital region, but a study of the lower vertebrates leads to the belief that they are present also in the anterior
part of the head, and that altogether nine segments are represented in the cephalic region.
SEPARATION OF THE EMBRYO.
The embryo increases rapidly in size, but the circumference of the embryonic disk, or line of meeting of the embryonic and amniotic parts of the ectoderm, is of relatively glow growth and gradually comes to form a constriction between the embryo and the greater part of the yolk-sac. By means of this constriction, which corresponds to the future umbilicus, a small part of the yolk-sac is enclosed within
the embryo and constitutes the primitive digestive tube.
The embryo increases more rapidly in length than in width, and its cephalic and caudal ends soon extend beyond the corresponding parts of the circumference of the embryonic disk and are bent in a ventral direction to form the cephalic and caudal folds respectively (Figs. 26 and 27). The cephalic fold is first formed, and as the proamniotic area (page 47) lying immediately in front of the pericardial
area (page 47) forms the anterior limit of the circumference of the embryonic disk, the forward growth of the head necessarily carries with it the posterior end of the pericardial area, so that this area and the buccopharyngeal membrane are folded back under the head of the embryo which now encloses a diverticulum of the yolk-sac named the fore-gut. The caudal end of the embryo is at first connected
to the chorion by a band of mesoderm called the body-stalk, but with the formation of the caudal fold the body-stalk assumes a ventral position; a diverticulum of the yolk-sac extends into the tail fold and is termed the hind-gut. Between the fore-gut
and the hind-gut there exists for a time a wide opening into the yolk-sac, but the latter is gradually reduced to a small pear-shaped sac (sometimes termed the umbilical vesicle), and the channel of communication is at the same time narrowed and elongated to form a tube called the vitelline duct.
The yolk-sac (Figs. 22 and 23) is situated on the ventral aspect of the embryo; it is lined by entoderm, outside of which is a layer of mesoderm. It is filled with fluid, the vitelline fluid, which possibly may be utilized for the nourislunent of the embryo during the earlier stages of its existence. Blood is conveyed to the wall of the sac by the primitive aortae, and after circulating through a wide-meshed capillary plexus, is returned by the vitelline veins to the tubular heart of the embryo.
This constitutes the vitelline circulation, and by means of it nutritive material is absorbed from the yolk-sac and conveyed to the embryo. At the end of the fourth week the yolk-sac presents the appearance of a small pear-shaped vesicle (umbilical vesicle) opening into the digestive tube by a long narrow tube, the vitelline duct. The vesicle can be seen in the after-birth as a small, somewhat oval-shaped body
whose diameter varies from 1 mm. to 5 mm.; it is situated between the amnion and the chorion and may lie on or at a varying distance from the placenta. As a rule the duct undergoes complete obliteration during the seventh week, but in about three per cent, of cases its proximal part persists as a diverticulum
from the small intestine, Meckel’s diverticulum, which is situated about three or four feet above the ileocolic junction, and may be attached by a fibrous cord to the abdominal wall at the umbilicus. Sometimes a narrowing of the lumen of the ileum is seen opposite the site of attachment of the duct.
THE term Embryology, in its widest sense, is applied to the various changes which take place during the growth of an animal from the egg to the adult condition: it is, however, usually restricted to the phenomena which occur before birth. Embryology may be studied from two aspects: (1) that of ontogeny, which deals only with the development of the individual; and (2) that of phylogeny, which concerns itself with the evolutionary history of the animal kingdom. In vertebrate animals the development of a new being can only take place when a female germ cell or ovum has been fertilized by a male germ cell or spermatozoon. The ovum is a nucleated cell, and all the complicated changes by which the various tissues and organs of the body are formed from it, after it has been fertilized, are the result of two general processes, viz., segmentation and differentiation of cells. Thus, the fertilized ovum undergoes repeated segmentation into a number of cells which at first closely resemble one another, but are, sooner or later, differentiated into two groups: (1) somatic cells, the function of which is to build up the various tissues of the body; and (2) germinal cells, which become imbedded in the sexual glands — the ovaries in the female and the testes in the male — and are destined for the perpetuation of the species. Having regard to the main purpose of this work, it is impossible, in the space available in this section, to describe fully, or illustrate adequately, all the phenomena which occur in the different stages of the development of the human body. Only the principal facts are given, and the student is referred for further details to one or other of the text-books on human embryology.
THE ANIMAL CELL.
All the tissues and organs of the body originate from a microscopic structure (the fertilized ovum), which consists of a soft jelly-like material enclosed in a membrane and containing a vesicle or small spherical body inside which are one or more denser spots. This may be regarded as a complete cell. All the solid tissues consist largely of cells essentially similar to it in nature but differing in external form. In the higher organisms a cell may be defined as “a nucleated mass of protoplasm of microscopic size.” Its two essentials, therefore, are: a soft jelly-like material, similar to that found in the ovum, and usually styled cytoplasm, and a small spherical body imbedded in it, and termed a nucleus. Some of the unicellular protozoa contain no nuclei but granular particles which, like true nuclei, stain with basic dyes. The other constituents of the ovum, viz., its limiting membrane and the denser spot contained in the nucleus, called the nucleolus, are not essential to the type cell, and in fact many cells exist without them.
Cytoplasm (protoplasm) is a material probably of variable constitution during life, but yielding on its disintegration bodies chiefly of proteid nature. Lecithin and cholesterin are constantly found in it, as well as inorganic salts, chief among which are the phosphates and chlorides of potassium, sodium, and calcium. It is of a semifluid, viscid consistence, and probably colloidal in nature. The living Citoplasm appears to consist of a homogeneous and structureless ground-substance in which are embedded granules of various types. The mitochondria are the most constant type of granule and vary in form from granules to rods and threads. Their function is unknown. Some of the granules are proteid in nature and probably essential constituents; others are fat, glycogen, or pigment granules, and are regarded as adventitious material taken in from without, and hence are styled cell-inclusions or paraplasm. Then, however, cells have been “fixed” by reagents a fibrillar or granular appearance can often be made out under a high power of the microscope. The fibrils are usually arranged in a network or reticulum, to which the term spongioplasm is applied, the clear substance in the meshes being termed hyaloplasm. The size and shape of the meshes of the spongioplasm vary in different cells and in different parts of the same cell. The relative amounts of spongioplasm and hyaloplasm also vary in different cells, the latter preponderating in the young cell and the former increasing at the expense of the hyaloplasm as the cell grows. Such appearances in fixed cells are no indication whatsoever of the existence similar structures in the living, although there must have been something in the living cell to give rise to the fixed structures. The peripheral layer of a cell is in all cases modified, either by the formation of a definite cell membrane as in the ovum, or more frequently in the case of animal cells, by a transformation, probably chemical in nature, which is only recognizable by the fact that the surface of the cell behaves as a semipermeable membrane.
Nucleus. — The nucleus is a minute body, imbedded in the protoplasm, and usually of a spherical or oval form, its size having little relation to that of the cell. It is surrounded by a well-defined wall, the nuclear membrane; this encloses the nuclear substance (nuclear matrix), which is composed of a homogeneous material in which is usually embedded one or two nucleoli. In fixed cells the nucleus seems to consist of a clear substance or karyoplasm and a network or karyomitome. The former is probably of the same nature as the hyaloplasm of the cell, but the latter, which forms also the wall of the nucleus, differs from the spongioplasm of the cell substance. It consists of fibers or filaments arranged in a reticular manner. These filaments are composed of a homogeneous material known as linin, which stains with acid dyes and contains embedded in its substance particles which have a strong affinity for basic dyes. These basophil granules have been named chromatin or basichromatin and owe their staining properties to the presence of nucleic acid. Within the nuclear matrix are one or more highly refracting bodies, termed nucleoli, connected with the nuclear membrane by the nuclear filaments. They are regarded as being of two kinds. Some are mere local condensations (“net-knots”) of the chromatin; these are irregular in shape and are termed pseudo-nucleoli ; others are distinct bodies differing from the pseudo-nucleoli both in nature and chemical composition; they may be termed true nucleoli, and are usually found in resting cells. The true nucleoli are oxyphil, i. e., they stain with acid dyes. Most living cells contain, in addition to their protoplasm and nucleus, a small particle which usually lies near the nucleus and is termed the centrosome. In the middle of the centrosome is a minute body called the centriole, and surrounding this is a clear spherical mass known as the centrosphere. The protoplasm surrounding the centrosphere is frequently arranged in radiating fibrillar rows of granules, forming what is termed the attraction sphere.
Reproduction of Cells. — Reproduction of cells is effected either by direct or by indirect division. In reproduction by direct division the nucleus becomes constricted in its center, assuming an hour-glass shape, and then divides into two. This is fol- lowed by a cleavage or division of the whole protoplasmic mass of the cell; and thus two daughter cells are formed, each containing a nucleus. These daughter cells are at first smaller than the original mother cell; but they grow, and the process may be repeated in them, so that multiplication may take place rapidly. Indirect divsion or karyokinesis (karyomitosis) has been observed in all the tissues — generative cells, epithelial tissue, connective tissue, muscular tissue, and nerve tissue. It is possible that cell division may always take place by the indirect method. The process of indirect cell division is characterized by a series of complex changes in the nucleus, leading to its subdivision; this is followed by cleavage of the cell protoplasm. Starting with the nucleus in the quiescent or resting stage, these changes may be briefly grouped under the four following phases (Fig. 2).
1. Prophase. — The nuclear network of chromatin filaments assumes the form of a twisted skein or spirem, while the nuclear membrane and nucleolus disappear. The convoluted skein of chromatin divides into a definite number of V-shaped segments or chromosomes. The number of chromosomes varies in different animals, but is constant for all the cells in an animal of any given species; in man the number is given by Flemming and Duesberg as twenty-four(?). (23 pairs). Coincidently with or preceding these changes the centriole, which usually lies by the side of the nucleus, undergoes subdivision, and the two resulting centrioles, each surrounded by a centrosphere, are seen to be connected by a spindle of delicate achromatic fibers the achromatic spindle. The centrioles move away from each other — one toward either extremity of the nucleus — and the fibrils of the achromatic spindle are correspondingly lengthened. A line encircling the spindle midway between its extremities or poles is named the equator, and around this the V-shaped chromosomes arrange themselves in the form of a star, thus constituting the mother star or monaster.
2. Metaphase. — Each V-shaped chromosome now undergoes longitudinal cleavage into two equal parts or daughter chromosomes, the cleavage commencing at the apex of the V and extending along its divergent limbs.
3. Anaphase. — The daughter chromosomes, thus separated, travel in opposite directions along the fibrils of the achromatic spindle toward the centrioles, around which they group themselves, and thus two star-like figures are formed, one at either pole of the achromatic spindle. This constitutes the diaster. The daughter chromosomes now arrange themselves into a skein or spirem, and eventually form the network of chromatin which is characteristic of the resting nucleus.
4. Telophase. — The cell protoplasm begins to appear constricted around the equator of the achromatic spindle, where double rows of granules are also sometimes seen. The constriction deepens and the original cell gradually becomes divided into two new cells, each with its ovii nucleus and centrosome, which assume the ordinary positions occupied by such structures in the resting stage. The nuclear membrane and nucleolus are also differentiated during this phase.