Low Ball Of Cells, Or Blastophere.

Fig. 2871. GASTRULA. D GROWTH OF ORGANS. The development of the embryo up to this point has been simple. In the later development of the body each germinal layer plays an active part. The ectoderm gives rise to the outer skin and its appendages, the external skeleton where present, the superficial glands the nervous system, . From the endoderm comes the visceral membranes, and their outgrowths, such as the lungs and various internal glands. In vertebrate animals it also gives rise to an important embryo structure, the notochord , which always precedes the development of the backbone. All the other organs of the body arise from the mesoderm, including the internal skin, the muscles, the internal skeleton, the connective ti ue, the lining of the body cavities, and the blood and its containing walls. The reproductive organs also usually arise in connection with the mesoderm. Some of the organs, however, have a compound origin. Thus the eye is an outgrowth from the brain, and therefore ectoderm in origin; but some of its structural parts arise from the mesoderm.Mammalian development. The phenomena attending the embryo development of the lower forms differ in many particulars, as the gastrula gradually takes on the special characters of the order and species, the general characteristics first appearing, and gradually the more special ones, ending with those of the race or species, and the individual. In the development of a mammal certain general steps are pa ed through in all cases; then le general ones appear; and what might have become any mammal, now takes on characteristics special to a more restricted cla , the proce gradually narrowing toward the special creature destined to appear. The germinal layers, once differentiated, begin quickly to display inequality of growth and specialization, yielding stage after stage of progre towards the final form. The embryonic area of what is to become a mammal, when looked upon from the surface, presents a somewhat clear central field, surrounded by an opaque peripheral section. From the caudal pole of the area there extends forward through the clear space, or area pellucide , a central opaque line, called the primitive streak. It is a temporary appearance, which forms no part of the embryo proper, though indicating where it is to form. The formation of the primitive streak is soon followed by that of two diverging folds, embracing the anterior part of the primitive streak. These ridges form the medullary folds , and the depre ion between them the medullary groove. At the bottom of the latter a rod-like layer of cells, the beginning of the notochord , soon forms. This marks out the line of the vertebral column, and the axis of the embryonic structure. Over the groove the folds in time unite, a canal or tube being formed, from which arises the nervous tract of the spine and brain. On either side segmental cleavages appear in the ti ue, called the somites ; temporary conditions, marking the places of future vertebræ. While these changes are taking place without, others are occuring within, including the formation of the primitive body cavity, by the separation of the mesodermic layers, and the formation of folds from which the body walls and, the visceral tubes are afterward produced. These are the general features of the early development of mammalian embryos. There are others relating to the development of special parts, such as the appearance of the gills-which precede the lungs of the higher forms-the outgrowth of the tail, the formation from the cerebral part of the neural tube of the rudimentary brain, and the outgrowth of the various temporary appendages of viviparous animals requisite to nutrition within the womb, -the amnion , the chorion , and their accomanying structures. What energies and internal influences are at work in all these stages of development is not easy to trace. Evidently special IMG:content-0107.png:[blocks in formation] Fig . 2872.-EMBRYOS. Comparing: a, fowl; b, dog; c, man. conditions of structure and energy exist in the germinal cell of every species of animal and plant which govern and control its development, for which only nutrition and suitable protection are nece ary, every step of the progre being governed from within, not only those general ones which mark out the broad features of organisms in general, but those which give rise to special organs, to family features, and to individual traits. These conditions must be very complex, minute as is the germ, and capable, by the aid of their inherent energies, and by directing the flow of each particle of nutriment, and its deposition in the requisite place, to carry out the plan of structure; while the brain organism must develop in such a manner as to provide conditions for the subsequent traits of mental disposition, of which the foundations are hereditarily laid. Consideration of this leads to questions which it is at present impo ible to solve, and with this brief statement of their character we may close.