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soft tissue called the decidua, within which the young ovum imbeds itself, and from which it at first derives its nutriment.

As the ovum grows, however, one portion of the decidua thickens and becomes highly vascular. This is called the decidua serotina (Fig. 149, m), and it forms a ring round that part of the uterine cornu in which the ovum lies. The adjacent part of the rapidly. enlarging ovum also becomes specially supplied with blood-vessels from the embryo contained within it, through the intervention of its already-mentioned allantois. Processes from the vascular ring of the chorion (Fig. 147, t) pass into recesses in the vascular ring of the uterus-the serotina decidua, and the two parts form an inseparable interlacement called the PLACENTA (Fig. 147, pl). The maternal and embryonic blood-vessels, however, nowhere actually communicate, and therefore no intermixture takes place between the blood of the embryo and the blood of the mother. An abundant gaseous interchange, however, is effected between them, the blood of the embryo taking oxygen from, and giving off carbonic acid gas into, the maternal blood. Thus the placenta is a temporary and indirect breathing organ for the embryo, which can breathe in no other way, lying, as it does, enclosed in fluid.

As has been already said, the allantois, with its blood-vesselscalled umbilical arteries and reins-stretches itself forth into and across the space within the ovum, till it arrives at the inner surface of the chorion, where it expands, wrapping round the whole embryo with its amnion, carrying its vessels to the vascular portion of the chorion, and so contributing to form the placenta. The junction once effected, the arteries and veins connecting the foetus with this part of the chorion rapidly enlarge. The umbilical arteries convey the impure blood of the foetus to the placenta, where it is purified, as before said, and nourished by the influence of the maternal blood (the two fluids having but thin membrane between them), and is then returned to the embryo by the umbilical veins, which proceed back along the allantois from the placenta. Meantime, while the embryo is thus enclosed in its amnion and nourished by its allantois, the splanchnopleure grows in on all sides in the way before mentioned, so as almost to separate the embryo from the remnant of the yelk in its sac-the umbilical vesicle. The stalk which connects this vesicle with the intestinal cavity of the embryo is short but slender. It is called the vitelline duct or ductus omphaloentericus. The umbilical vesicle remains of rather large size as a transversely-elongated sac produced into two horn-like prolongations. It lies between the amnion and the allantois.

The structure which connects the embryo or foetus with the placenta is called the umbilical cord. This is made up mainly of the narrow part of the allantois (consisting of the embryonic or foetal arteries and veins with connective tissue) with the vitelline duct, the whole being bound round and enclosed by a fold of the amnion extending down round it, and being thence reflected over the foetus (Fig. 147, tc). The umbilical cord is the sole channel

through which nourished and oxygenated blood is conveyed to the foetus, and its importance continues till birth. The establishment of pulmonary respiration, however, and the acquisition of the power of taking nourishment by suction, do away with all need for the placenta, and, by consequence, for its stalk, the umbilical cord, which is gnawed across by the mother on the kitten's birth. The

Fig. 150.-FTUS IN UTERO (BUFFON AND DAUBENTON), SHOWING THE FETUS ENVELOPED IN ITS
AMNION, AND WITH ITS ZONARY PLACENTA AND SPINDLE-SHAPED UMBILICAL VESICLE.

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part left in connexion with the abdominal wall soon shrivels up, dries, and falls off, but a permanent mark of its place of attachment persists throughout life as the umbilicus or navel.

Such being the form and arrangement of the foetal membranes and adjuncts, the embryo or FOTUs itself gradually and in a roundabout way assumes the image of the kitten in the mode already intimated, and which will be more fully explained in describing the development of the several organs.

The period of gestation is fifty-five or fifty-six days. The ovum having by that time attained its full inter-uterine development, vigorous contractions of the muscular walls of the uterus ensue, while the os uteri dilates. The embryo is thus expelled from the

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uterine cavity, and comes away, bringing with it a portion of the maternal part of the placenta, together with the foetal part, with which the maternal part is inextricably united. Thus the superficial part of the decidua serotina comes away, while its deeper part is left, with a torn and bleeding surface. Coincidently with the termination of gestation, the mammary glands take on functional activity and become ready to play their part in the post-natal development of the young. After the expulsion of the embryos, of which several are in general simultaneously developed, the uterus forms a fresh internal lining, while the thickness of its walls decreases by degeneration and absorption of a portion of its muscular tissue, which had so much increased in quantity during pregnancy. § 7. It remains to consider seriatim, the development of the various TISSUES and ORGANS of the body.

All the various tissues and structures of the adult cat (connective tissue, cartilage, bone-all parts of the skeleton of course included) arise from the primitive fluid, granule, and cells of the fertilized ovum by a process which is called differentiation. This term, which is often used as if it were a real explanation, simply denotes the fact that the various parts arise not through external actions, which are but the concomitants and conditions of their origin, but by an as yet utterly inexplicable and innate power possessed by the primitive substance or matrix, within which the parts referred to, come (under the requisite external conditions) gradually to manifest themselves. Other terms relating to development similarly denote spontaneous and mysterious actions of the formative power, and are but convenient phrases for denoting the actions of such power, and not explanations of it.

Such terms, for example, are segmentation—which denotes that a structure, primitively of one piece, spontaneously divides its substance into parts; vacuolation-denoting the spontaneous resolution of part of a more or less dense structure in such a way as to give rise to a cavity or cavities within it; and fenestration-denoting that a solid structure has dissolved itself at one spot or more, so as to give rise to an aperture perforating it. When then the development of the tissues and parts of the body are herein described, the intention is but to state the order and mode in which they manifest themselves, the fact being distinctly recognized that an innate force is the real and efficient cause.

The primitive almost fluid substance containing granules, which exists in the developing ovum, is known as protoplasm; and protoplasm is often spoken of as if it were a sort of primary organic material -a distinct kind of formed substance-from which all organisms arise. But the fact that the primitive substance of one animal or plant is not to be distinguished by any chemical or physical test we can apply from the primitive substance of another animal or plant, does not by any means prove that the two are really the same substance. It does not prove this identity, because of the very different results which are successively evolved in the two cases, as development proceeds.

This allimate diversity is amply ficient to show that 3 Tel Be ence existed from the first-a difference this demonstrable reason, though not manifest to our senses.

From the primitive substance of which the car's wan Prusists, il the ultimate constituent parts of its body are derived them the hels of the cell formations, already deserted as the epitus, bajili, and mesoblast. It has already been mentioned that the epillist CITES rive to the eriders of the skin and to the nervios centres; the hypoblast, to the alimentary epithelium: and the mesoclast. 1. the great mass of the body. But parts which are derived from one of these sources may acquire characters quite like those derived from an then. Thas the linings of the two ends of the alimentary mail are as Las been sall Ermed from infected epiblast, ani epithecal structures as in the lining of the vessels and of the perit cell any can be formed as well from the mesoblast as from the excllast.

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All the tissues and organs of the cat's body are then derived from cells, and indeed they are doubly so derived, since the before jelk segmentation begins, is a perfect e-ai is el for periplast and its wel render the latter being for nished with one or more wwcles. This this all begs the als of the three layers of the embryo, an i these latter cells begut all the tissues and organs which subsequently arise, and the great mass of them are begotten by the cells of the mesoblast.

But though all the tissues have this ultimate sellir parentage, they by no means always retain a plainly cellular structure, as they severally arise from the primitive, or “in femet" tissue of the mesoblast, and become definite connective, muscular or Devous substance, as the case may be. Sometimes they take on the form simply of a soft substance of one or another kind, within which nuclei are embedded at intervals. From analogy we may regard the parts of such substance which are adjacent to such nudei as representing cells, the limits of which are severally in distinguishake.

The five main constituents of the cat's body- I connective tissue, with its derivatives, cartilage and bene; 2 epithelial tissue; 3; Blood: 4 musealar tissue; and 5) nervous tissue-arise as follows:

Conective tissue appears to arise in the embry, partly as a flylike substance, or matrix, and partly as ells from the mes Clas which though more or less separated by this substance, yet remain connected by processes which grow out in a radiating manner from them. The fibres of the tissue are by some observers deserted as arising within the protoplasm of the cells, those of adjacent oils uniting, while the parts of the cell not thus transformed persist as e treetive-tissue corpuscles. Other observers, however, believe that the thres arise, as an indepen lent deposit, within the intercellular substance. Estic tissue is said to be formed from other cells which grow out and branch, becoming connected with processes from neighbouring alls Cartilage appears in its simplest condition in the charit des or notochord) as a mass of closely applied, thin-walled cells The layer of the embryo from which these are derived, is as has been

said) not yet positively ascertained, but the other cartilages of the body are undoubtedly of mesoblastic origin. The walls of the primitive cartilage cells thicken and form the intercellular matrix, acquiring at the same time the special qualities of cartilaginous tissue. The thus-formed matrix may remain clear and structureless, or may become fibrillated, as in the case of fibro-cartilage.

Bone is a substance which is never directly formed from the indifferent embryonic tissue, but requires for its development the pre-existence of either cartilage or connective tissue. The process of its formation in these substances has been already noticed.*

Epithelial tissue is the most distinctly and permanently cellular of all the tissues of the body, and it arises directly from the cells of the epiblast and hypoblast, with the exception of the endothelium of the vessels and pleuro-peritoneal cavity, and some other parts which are derived (as already said) from the mesoblast. The ependyma of the cerebral ventricles is the persistent epiblast of the lining of the medullary groove of the embryo. Ova and spermatozoa may be considered as special modifications of epithelial tissue.

Blood appears to originate within cells derived from the mesoblast,† either by a multiplication of their nuclei and the acquisition. of a red colour by the protoplasm around each nucleus, or else as a sort of deposit within the cells. However originating, the primitive corpuscles when formed become separated from one another by a process of vacuolation within the cells. The cells then enlarge and send out processes which unite with those of other cells. The walls which separate the cavities of such united processes then disappear, so that their cavities communicate, and thus blood and bloodvessels are simultaneously formed.

The primitive red corpuscles are nucleated, and larger than those which subsequently arise, which latter, together with the white corpuscles, seem to be formed mainly by the spleen, the nucleated red corpuscles disappearing and being replaced by the smaller, flattened non-nucleated corpuscles, during embryonic life.

Muscular tissue, though by no means clearly cellular in its fully formed condition, is said to have a distinctly cellular origin, being formed by direct transformation of embryonic cells as follows: § the cells elongate and acquire an investing membrane and often pointed ends. The nuclei multiply, and the contained protoplasm of the cells gradually acquires its striated character. The investing membrane becomes the sarcolemma, and the scattered nuclei become the corpuscles. The unstriped muscular fibres originate simply by the lengthening out and flattening of cells, which acquire pointed ends and an elongated nucleus.

Nervous tissue is, as has been said, mainly derived from the epi

*See ante, p. 20.

A perfect agreement has not yet been arrived at as regards their mode of origin. See Balfour, Quarterly Journal of Microscopic Science, July, 1873 and

Schäfer's Proceedings of the Royal
Society, 1874.

See ante, p. 329.

§ See Wilson Fox, Phil. Trans., 1866.

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