which here undergoes an hypertrophy; it is moreover characterized by the presence of both vascular and ramified villi. These villi originating on either membrane are joined together, interlace, and, finally, form the more or less circular and apparently compact cake, which becomes the centre of exchange between the maternal and foetal organism (Fig. 143, 20).

Fig. 144.-Diagram of the placental vessels.*

An idea of the method of interchange between the mother and foetus is represented by a diagram in Fig. 144. The fœtus receives and rejects the nutrient materials by means of an osmotic interchange through the capillaries of each villus; this constitutes nutrition and respiration.

The foetal respiration is effected by means of the placenta; we have already spoken of this form of respiration (see,p. 324). The necessity of placental respiration is, moreover, supported by the serious accidents which result from suppression of the placental functions. When the circulation of the cord which unites the placenta to the fœtus (see fœtal circulation) is interrupted, the fœtus perishes, not so much through want of nourishment, as from a true asphyxia; at birth pulsations in the cord cease only when the infant respires through the lungs, because then the new method of respiration definitely replaces that which has been accomplished by the utero-placental connection.

The nutrition of the fœtus during the placental portion of its life consists of an interchange of materials between the fœtal and the maternal blood through the placenta. Moreover, the relations which combine both nutrition and respiration are much simpler in the fœtus; the adult consumes materials and transforms them into work (see Mechanical equivalent of heat, p. 78) or heat. The fœtus has to perform no

*1, Uterus. 2, Intermediate tissue. 3, Placenta (membrana reflexa seu caduca serotina), where the maternal and foetal vessels ramify. (ChaillyHonoré.)

work and expends no force; it has not even to produce heat, it receives that from the mother. It takes alimentary materials only for the building of tissues and the development of organs; consequently the difference in the character of its venous and arterial blood is not very great, and by no means the same as in the arterial and venous blood of adult life. However, oxidation, no matter how feeble it may be, is produced in the embryo; thus, the heart performs its work and must occasion products of combustion; moreover, every formation of tissue is accompanied by phenomena of combustion, which should give rise to excrementitial products. These products are eliminated principally by the liver and urinary organs (first the Wolffian body and then the kidneys); the liver is also very much developed in the embryo, and up to a certain point it may replace the lung as an organ for the excretion of organic waste. A certain amount of urea is also contained in the bladder of the embryo, which is thrown off into the amniotic cavity. Consequently, the amniotic fluid contains at the close of the embryonic life a large number of excrementitial products, because in addition to the urine there are products resulting from the desquamation of the skin.

II. Development of the body of the embryo.

If we bear in mind what has preceded in regard to the formation of the umbilical vesicle (pp. 500 and 503) we shall also understand how this vesicle, in consequence of a peculiar strangulation, is separated from the common blastodermic vesicle (p. 587); the borders of the germinal space or area, as well as its cephalic and caudal extremities or hoods, drawn along by this strangulation or constriction, form by their curvatures the sides as well as the cephalic and caudal hoods (Fig. 137, 139, 140), which unite and form a cavity. This cavity might be likened to the hollow or interior of a slipper, and communicates with that of the umbilical vesicle, as we have before stated (Fig. 139, p. 501). This is the primary cavity of the embryo, or rather its intestinal cavity (Fig. 137, 12). To complete this rough sketch of embryology we will proceed to the consideration of the two grand systems, the nervous system and that of the circulation.

a. Central Nervous System. As soon as the germinal space or area has assumed the form of an elongated spot (like the sole of a slipper) a central longitudinal line, called

the primitive groove, appears; this serves as the point of origin of the central nervous system (spinal cord and encephalon). In fact, this line is simply a groove (Fig. 145) bounded by two longitudinal ridges of the external layer or fold (epiblast) of the blastoderm. These two ridges (medullary folds, Fig. 145, 3) extend backwards and, by their union, surround the medullary canal. This canal is represented in the adult by the central canal of the spinal cord with the fourth ventricle and the ventricles of the brain (and the aqueduct of Sylvius). The incomplete closure of this medullary groove results in the formation of the fourth ventricle. It is generally admitted at

the present time that the external layer of the blastoderm (epiblast) forms only the epithelium of the central canal of the spinal cord (and cerebral ventricles, vibratile epithelium, see p. 190), and that the nerve elements originate in a part of the middle layer (mesoblast) subjacent to this epithelium. This view is confirmed by the fact that everywhere else the nerve elements are formed at the expense of the intermediate layer (mesoblast).

The upper portion of the medullary or neural canal forms the encephalic substance; this part swells out into three vesicles (cerebral vesicles or cells) which are respec

5

4

3

2

6

system.*

dle cerebral, and the posterior cerebral cells, or the first, second, and third cerebral vesicles. The anterior or first cerebral cell or vesicle is again divided into two portions, the most anterior of which (anterior of the brain), overlaying the second, forms the cerebral hemispheres and corpus callosum, and the posterior (intermediate portion of the brain) forms the thalami optici and the third ventricle (continuation of the medullary canal): 2. The middle or second cerebral

* 1, Medullary groove. 2, Inferior enlargement of the medullary groove (rhomboid sinus). 3, Crests or medullary folds (laminæ dorsales). 5, Middle and external folds of the blastoderm. 6, Inner fold of the blastoderm (Bischoff).

vesicle is not divided (middle portion of the brain) and forms the region of the corpora quadrigemina with the aqueduct of Sylvius (continuation of the medullary canal); 3. The posterior or third cerebral vesicle divides, like the first, into two portions; one of which, that nearest to the middle part of the brain, will form the protuberance, or medulla oblongata, and the cerebellum (posterior part of the brain); the other of these divisions, a direct continuation of the spinal cord, will form the rachidian bulb (medulla oblongata, strictly speaking); this is the point in which the medullary or neural canal does not completely close, but persists in its original form of a groove and constitutes the floor of the fourth ventricle.

The peripheral nerves are formed in their proper place at the expense of the middle layer (mesoblast) of the blastoderm. The optic nerve and retina form an exception and are represented by a diverticulum of the encephalic substance (see p. 425, Fig. 113.)

The ganglia of the great sympathetic are also formed in their proper places, independently of the cerebro-spinal substance, and from the middle layer (mesoblast) of the blastoderm, as we have already learned, in treating of the semilunar ganglia of the abdominal portion of the sympathetic system (see p. 277).

b. Circulation in the Embryo. - The circulation of the embryo depends upon its method of nutrition. As we have already learned, this nutrition of the embryo may be effected in three different ways: First, by the simple and direct assimilation of the albuminous substance in which the ovum is immersed; no system of circulation is required for this simple form of imbibition. Second, by an assimilation of the contents of the umbilical vesicle; these contents are conveyed to the embryo by a peculiar system which forms the primary or omphalo-mesenteric circulation (sometimes written omphalo-mesaraic). Third, by an interchange with the maternal blood through the placenta; this method of nutrition is fulfilled by the secondary or placental circulation.

1. The system for the primary circulation commences with the formation of the heart; this organ is at first represented by a cylinder of embryonic globules; soon the surrounding globules become organized into muscular fibres, whilst those at the centre undergo a partial dissolution and form the first blood. Simultaneously the heart, which at first

was longitudinal, now assumes the form of the letter S (Fig. 146, 4), and commences to contract and propel the blood into the peripheral vessels.

These peripheral vessels, as we have already said, are formed in their proper places and consist, at the first, of two aortic arches, which are offshoots from the anterior extremity

of the cardiac tube. These curve around and below the cephalic hood (anterior vertebral arteries), unite in a single trunk (aorta) at the median portion of the vertebral column, and again divide, descending towards the caudal extremity of the embryo by two branches, the posterior vertebral;

Germinal area of an embryo; the ventral surface of the embryo is presented. 1, Terminal sinus. 2, Omphalo-mesenteric vein. 3, Its posterior branch. 4, Heart in the form of an S. 5, Primitive aorta, or posterior vertebral arteries. 6, Omphalo-mesenteric arteries. (Bischoff, Developpement de l'Homme," p. lxiv.)

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