Reproduction 257 



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

 sauropsidan 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 saurop- 

 sidan embryo begins at a point which corresponds very closely to the 

 position of the primary gastrular invagination in the amphibian (Fig. 

 211 A', I). This fact, together with later events in the sauropsidan 

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

 aperture of the little invagination or the slit formed by infolding of 

 the hind edge of the blastoderm. 



Comparisons. Comparison of the early development of Amphi- 

 oxus, 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 am- 

 phibian with total but very unequal cleavage, the reptile or bird with 

 partial cleavage, the several embryos at corresponding stages exhibit- 

 ing radical differences in the configuration of their materials — yet 

 analysis of the processes concerned in the development 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 be- 

 cause of variation in yolk volume. The method whereby the embryo 

 of a reptile or bird 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 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. 



There is reason to believe that the Amphioxus embryo with its 

 nearly minimum encumbrance of yolk reveals, at least in the early 

 stages of development, the primitive and basic processes which, in 

 most other chordates, become more or less obscured by yolk. 



