70 DEVELOPMENT OF THE FROG'S EGG [Cn. VI 



mesodermal and endodermal cells as being together from the 

 beginning. 



Different Accounts of the Origin of the Archenteron 

 and Mesoderm 



Before following further the fate of these concentric coats 

 or layers of cells, the so-called " germ-layers," we may for a 

 moment examine some other descriptions that have been given 

 as to the method of formation of the archenteron in the frog. 

 The most common view of the method of gastrulation of the 

 frog has been that a process of invagination takes place at the 

 dorsal lip of the blastopore. This process is supposed to be 

 brought about by the drawing inwards and upwards of a fold 

 of the outer wall, so that a blind sac forms. As this presses 

 forward into the yolk, the latter pushes before it and fills up 

 the segmentation-cavity. At the same time the mesoderm is 

 described as growing forward from the region of the blastopore 

 over the dorsal surface of the embryo. 



Other authors represent, however, the dorso-lateral edges 

 of the archenteron proliferating cells along the two sides to 

 form the mesoderm, while in the mid-dorsal line a solid block 

 of endoderm cuts off to form the notochord. Hertwig has gone 

 so far as to affirm that at the dorso-lateral edges of the archen- 

 teron there are traces of a pair of lateral pouches along each 

 side, and that these give rise to the cells that push in between 

 the ectoderm and endoderm to form the middle layer. 



Robinson and Assheton ('91) assert that the old account of 

 the formation of the archenteron by invagination is entirely 

 erroneous, and that the cavity of the archenteron owes its exist- 

 ence to a process of progressive splitting or separation of the 

 large yolk-cells of the lower hemisphere, and that this splitting 

 extends up into the yolk beneath the upper hemisphere. The 

 dorsal lip of the blastopore remains approximately stationary 

 where it first formed, and the anus develops around this point. 1 



1 In a later account Assheton ('94) has much altered his former view. He 

 describes only the anterior end of the archenteron as formed as a split amongst 

 the endoderm- cells, while the posterior third of the archenteron is, he thinks, the 

 result of the overgrowth of the dorsal and lateral lips of the blastopore. 



