232 BIOLOGY: GENERAL AND MEDICAL 



This didermic plate, consisting of ectoderm and ento- 

 derm, between which the cells of the mesoderm soon 

 make their appearance, is not essentially different from 

 the oval germinal disc floating upon the great yolk of the 

 hen's egg, and its future development progresses in much 

 the same way. In the mammalian egg, however, the 

 researches of Peters and van Beneden indicate that 

 endoderm formation does not take place through gas- 

 trulation or invagination as in the eggs of the lower 

 phyla, but, like the amnion formation, is arrived at by a 

 short cut i.e., by vacuolization of certain cells. 



After the formation of the embryonic area (germ disc) 

 composed of its three germinal layers, the future develop- 

 mental process consists in a succession of wrinkles and 

 puckers resulting from unequal growth of cells in different 

 locations, a general tendency of the external convex 

 surface to outgrow and surround the inner concave 

 surface, the fusion or concrescence of many of the folds, 

 the growth of groups of cells at certain points to form 

 organs and so bring about the final evolution of the 

 fostus. 



The external morphological development and the 

 resemblances it embraces can be more quickly understood 

 by an examination of the accompanying illustrations 

 taken from Romanes than from a lengthy description. 



Internal development progresses simultaneously with 

 external development. Perhaps so much of this has 

 been left to the imagination of the reader that it may be 

 well to consider certain features more particularly. 

 Let us begin by remarking that it progresses simply or 

 complexly according to the future simplicity or complex- 

 ity of the species to which the embryo belongs. Let it 

 also be understood that in spite of complexity it pro- 

 gresses rapidly, and that the complexity incidental to 

 the phylogenetic position of the animal whose embryo 

 is examined is not the result of the consecutive forma- 

 tion of the internal organs, but of their simultaneous 

 formation. 



