70 COMPARATIVE ANATOMY 



It is noteworthy that the place of origin of the endoderm in the 

 sauropsidan embryo is always at the edge of the blastoderm and it is 

 always the posterior edge. If the primary blastoderm is to be regarded 

 as corresponding to the animal hemisphere and the yolk mass to the 

 vegetal hemisphere of the amphibian embryo, then the formation of endo- 

 derm in the sauropsidan embryo begins at a point which corresponds very 

 closely to the position of the primary gastrular invagination in the amphib- 

 ian (Fig. 44.4', /). This fact, together with later events in the sauropsidan 

 embryo, justifies the application of the term, blastopore, to the aperture 

 of the Httle invagination or the slit formed by infolding of the hind edge 

 of the blastoderm. 



In the development of the reptile and bird, therefore, there is a gastrula 

 stage which, in spite of the enormous mass of yolk, is essentially similar 

 to the gastrula of an amphibian. It seems fair to say that the process of 

 sauropsidan gastrulation is as nearly as possible — that is, as nearly as the 

 yolk will permit — like that of an embryo in which there is comparatively 

 little yolk (see Fig. 81). 



Comparisons. Comparison of the early development of Amphioxus, 

 amphibian and reptile or bird compels the conclusion that, were it not for 

 difference in volume of yolk, the several embryos would be practically 

 alike in form, at least through the gastrula stage. It is as if the embryo 

 with the larger yolk mass " tried" to behave like the embryo of Amphioxus 

 but is compelled by the yolk to modify its behavior. Amphioxus with total 

 and nearly equal cleavage; the amphibian with total but very unequal 

 cleavage; the reptile or bird with partial cleavage; the several embryos at 

 corresponding stages exhibiting radical differences in the configuration 

 of their materials — yet analysis of the processes concerned in the develop- 

 ment of all these animals reveals a basic similarity. 



The actual animal is the protoplasm. Developmental processes are 

 its dynamic expression. Yolk, although necessary, is mere inert luggage. 

 In all these animals its composition is essentially the same. The similari- 

 ties which exist in spite of variation in yolk volume are certainly much 

 more significant than the differences which exist because of variation in 

 yolk volume. The method whereby the sauropsidan embryo achieves a 

 two-layered condition is not the simplest imaginable. The easy and direct 

 way would consist in the splitting of the original blastoderm to form two 

 layers, an inner and an outer. Such splitting or " delamination " of layers 

 commonly occurs at other stages in development. The fact that the 

 sauropsidan embryo initiates endoderm formation by invagination or 

 infolding at the posterior edge of the blastoderm is open to no better 

 explanation than that there is some necessity of adhering as closely as 

 possible to the developmental methods employed by amphibians and 

 Amphioxus. Such necessity can come only through inheritance. The 



