Page:Popular Science Monthly Volume 84.djvu/537

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lar rod (Fig. 12, ch), which then in higher vertebrates becomes surrounded successively by a fibrous, a cartilaginous, and a bony sheath. And so one might go on with a description of all the organs of the body,

PSM V84 D537 Facts and factors of development fig13.jpg

Fig. 13. Four Cleavage Stages of the Sheep; pb, polar bodies. (After Assheton.)

each of which begins as a relatively simple group or layer of cells, which gradually become more complicated by a process of growth and differentiation, until these various embryonic organs assume more and more the mature form.

6. Oviparity and Viviparity.—This very brief and general statement of the manner of embryonic development applies to all vertebrates, man included. There are many special features of human development which are treated at length in works on embryology, but which need not detain us here since they do not affect the general principles of development already outlined. In one regard the development of the human being or of any mammal is apparently very different from that of a bird or frog or fish, viz., in the fact that in the former the embryonic development takes place within the body of the mother whereas in the latter the eggs are laid before or soon after fertilization. In man, after the cleavage of the egg, a hollow vesicle is formed, which becomes attached to the uterine walls by means of processes or villi which grow out from it (Fig. 14, D, E, F) while only a small portion of the vesicle becomes transformed into the embryo. There is thus established a connection between the