FACTS AND FACTORS OF DEVELOPMENT 



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lar rod (Fig. 12, ch), which then in higher vertebrates becomes sur- 

 rounded 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, 



Fig. 13. Four Cleavage Stages of the Siieep ; 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 differen- 

 tiation, 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 



