REPRODUCTION 49 



The endoderm, thus initiated, rapidly spreads over the yolk-mass and 

 under the original layer which is now identified as the ectoderm. The 

 growth of the endoderm may be augmented by cells which become 

 detached from the under surface of the outer layer. 



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

 sidan embryo is always at the posterior edge of the blastoderm. 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 endoderm in the sauropsidan embryo 

 begins at a point which corresponds very closely to the position of the 

 primary gastrular invagination in the amphibian (Fig. 40,!', /). This 

 fact, together with later events in the sauropsidan embryo, justifies the 

 application of the term, blastopore, to the aperture of the little invagina- 

 tion or the slit formed by infolding of the hind edge of the blastoderm. 



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 similarities 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 Third Layer, Mesoderm 



The greater extent of the ectoderm of the embryo persists as the 

 essential layer, epidermis, of the adult skin. The endoderm gives rise 



