So COMPARATIVE ANATOMY 



nism of the germinal protoplasm. That mechanism can be changed, but 

 there is a high degree of inertia about it. As the vertebrate egg acquired 

 a bulky yolk mass certain adaptive changes in developmental methods 

 were inevitable. But the primitive methods were so far as possible 

 retained even at the expense of compelling the embryo to adhere to some 

 processes which, under the new circumstances, are indirect and unneces- 

 sarily complicated — as when the sauropsidan endoderm arises by infolding 

 at the edge of the blastoderm rather than by some simple and direct 

 process such as delamination from the entire blastoderm. Again, in 

 connexion with the reduction of yolk volume in eggs of recent mammals 

 the developmental mechanism inherited from reptiles has undergone 

 relatively little change beyond what has been immediately forced upon 

 it by absence of the yolk mass and, secondly, by early attachment to the 

 uterine wall in connection with the estabhshing of the placenta. 



The initiation of evolutionary change is evidently not within the 

 embryo itself. The embryo — that is, the living protoplasmic thing, 

 exclusive of inert yolk — is highly conservative. Its inertia is such that 

 it tends always to follow the old methods and it changes only as it must. 



In striking contrast to this evolutionary conservatism of the embryo 

 is its capacity for prompt adjustment to immediate emergencies. If 

 one cell of the two-cell cleavage stage of Amphioxus or an amphibian is 

 removed or so damaged as to prevent its development, the remaining 

 cell may produce a complete embryo nearly or quite normal in every 

 particular except size. A large portion of the early" blastoderm of a fish 

 egg may be destroyed and yet the remaining fraction of it will develop 

 into a normal young fish. 



Organogenesis 



Appreciation of the processes whereby a relatively small spherical 

 body of not visibly differentiated protoplasm becomes a large animal 

 having the complex body-plan of a vertebrate requires a knowledge of 

 the early embryonic stages, and especially the origin of germ layers and 

 their characteristic configuration at the time when the shaping up of 

 special organs is about to set in. Given the embryos of vertebrates of the 

 several classes, the three germ layers having been fully established, the 

 development of any major organ proceeds with a high degree of uniformity 

 in all the classes. In view of these facts, the foregoing account of early 

 development has been somewhat detailed while the following description 

 of the embryonic origin of the more important organs can be relatively 

 brief and general. These descriptions will be supplemented, too, in the 

 later chapters dealing with the comparative anatomy of the several 

 systems of organs. 



