LECTURE III 



The conditions in the Reptilia are in some respects intermediate 

 between those of the Amphibia, Birds and Mammals. In each 

 case there is no early formed blastopore and the archenteron is 

 formed by the accumulation or secretion of fluid among the yolk 

 cells, that is to say among the endoderm cells. 



This is very clearly shown in Will's figures of Platydactylus. 

 There are the same primary and secondary growth centres at 

 work, but the conditions under which they work are quite different. 

 The difference in the conditions which determine the difference 

 in effects seems to be (i) the less abundant, and less rapid accumula- 

 tion of fluid or perhaps it would be more accurate to say the less 

 early accumulation or secretion of the fluid among the endoderm 

 or yolk cells; (ii) the form of the secondary "centre" is discoid 

 instead of annular. The condition is intermediate between that 

 of an Amphibian like Hypogeophis, and a Mammal, as Fig. 26 

 will illustrate. 



There would seem to be an accumulation of fluid among the 

 endoderm cells in Hypogeophis, but this never causes a swelling 

 because the space wherein it accumulates opens at once to the 

 exterior by the "blastopore" shown in Fig. 27. 



In the Bird and the Mammal, as we have seen, the fluid 

 accumulates very rapidly and there is no way for its escape, so 

 the cavity enlarges, becoming the subgerminal cavity in the one 

 case, and the blastocyst cavity in the other. In both cases it 

 eventually forms the anterior part of the gut cavity. It can 

 therefore be called the archenteron. 



In the Reptile there is an undoubted accumulation of fluid as 

 in the Mammals and Birds and there is at this time no blastopore 



