6o J EN KIN SON, Germinal Layers of Vertebrates. 



retain a connection, in the primitive plate, with the cells 

 of the upper layer. In the higher forms — the Birds and 

 Mammals — this similitude of the paraderm to Amphibian 

 yolk-cells is lost, as well as the connection between the 

 two layers in the primitive plate. The upper layer fuses 

 only secondarily with the paraderm when the primitive 

 groove and streak are developed. 



There still remains an accessory problem. What is 

 the relation between the Gymnophiona and the rest of 

 the Anamnia ? The Gymnophionan blastopore though 

 apparently precisely similar to that of the frog corresponds, 

 as we have seen, only to a part of the latter. It is clear, 

 therefore, that the former must be derived from a blasto- 

 pore in which the anterior edge of the blastoderm takes 

 no part and which is consequently devoid of a ventral lip. 

 And very fortunately such a blastopore is to be found in 

 a form which not only is less megalecithal than the 

 Gymnophiona, but which may be presumed to stand not 

 far from the direct line of Amphibian descent ; I refer, of 

 course, to Lepidosiren {Fig, 33, B), which may be taken 

 back in its turn, if necessary, to Petroviyzon. The increase 

 of yolk that has taken place between the Dipnoi and the 

 Gymnophiona has simply resulted in the restriction of the 

 process of germ-layer formation to a perpetually diminish- 

 ing region of the blastoderm placed symmetrically about 

 the posterior margin or dorsal lip {Fig. 33, C). These 

 lateral lips — fusing behind in a ventral lip — have then 

 produced a circular blastopore superficially resembling 

 that of the remaining Amphibia {Fig. 33, A). The relation 

 between the two is, of course, just that which Balfour 

 supposed to exist between the primitive groove of the 

 Amniota and the Anamnian blastopore ; but it hardly 

 needs, I hope, to be pointed out that the present theory 

 differs ioto coelo from his ; for the representative in the 



