STUDIES IN MAMMALIAN EMBRYOLOGY. 537 



attempts at harmonious interpretation of the gastrulation 

 phenomena in the Amniota. 



A hypothetical intermediate stage may serve this purpose. 

 It is given in the diagram of fig. 93. It may be derived from 

 fig. 92 by supposing the ventral wall of this latter stage to 

 bulge out into a more capacious reservoir for the retention 

 either of nutritive yolk or of elastic fluid contents, as was 

 already noted above (p. 528) . If this process of development 

 takes place at the ventral pole of the spherical blastocyst, then 

 the ventral hemispherical cap of hypoblast of fig. 92 which in 

 the Cyclostomata and Amphibia gives rise to the lateral and 

 ventral walls of the intestine, will open out in trumpet fashion, 

 as indicated in fig. 93. The additional sac-like reservoir will 

 be a medio-ventral appendage to this intestine, and the hemi- 

 spherical zone will have become annular, as is indicated in semi- 

 perspective in fig. 93. 



The regions which in fig. 92 were marked by the dark blue 

 dots will retain their position, and so will the layer hy' . that 

 temporarily forms the floor of the archenteric cavity of invagina- 

 tion. It should be borne in mind that also in Amphibia and 

 Sauropsida this floor is only a temporary one, that it dwindles 

 away as the so-called Dotterpropf is being resorbed, and that it 

 is then finally replaced by the definite floor of the intestine 

 which has developed out of the hypoblast-cells that ab origin e 

 occupied the lower surface of the hemispherical cap (= the 

 annular zone of fig. 93). 



If the transitory character of this layer hy' ., which is indis- 

 putable in the lower Ichthyopsida, is retained in the hypo- 

 thetical stage of fig. 93 and further in the mammalian (and 

 sauropsidan) development, we shall have to look for it just below 

 the protochordal wedge and the front end of the gastrula ridge, 

 these being the coinciding regions that are stretched out above 

 it. Now in Mammalia such a portion of hypoblast does exist, 

 and coalesces with the cells of gastrula-ridge and protochordal 

 wedge. Centrifugally it merges into those hypoblastic surfaces 

 which actively contribute to the formation of what will ulti- 

 mately be the lateral and ventral wall of the intestine. So in 



