Formation of the Gut Cavity 15 



Thus in Rana, Siredon, and Hypogeophis one sees very plainly 

 from anatomical observations that the gut cavity is due to two 

 processes. 



A point to be noted is that when as in Amphioxus and Rana 

 there is a real segmentation cavity, namely a cavity produced 

 by the geometrical arrangement of the early segments, then this 

 cavity, as in Amphioxus, is roofed over by cells which will 

 eventually become ectoderm cells, and its floor is formed by cells 

 which will eventually become endoderm cells. 



Before it becomes converted into gut cavity there is a movement 

 of segments so as to line the ectodermic roof. This movement 

 in part is no doubt due to pressure caused by the expansion of 

 the gut cavity, though partly perhaps to the pressure produced 

 by the growth backwards of the blastopore lips. It can be taken 

 as representing a modified form of invagination. 



Invagination such as one sees in a yolkless egg (Amphioxus) 

 is impossible therefore gastrulation is affected by some other 

 means. 



It is doubtful to what extent the gut cavity is due to the 

 splitting referred to above. 



This is rather an important matter; because all the part of 

 the gut cavity which is produced either by splitting or by any 

 modification of invagination, or by conversion of the segmentation 

 cavity (so catted) clearly differs from that which is produced by 

 the GROWTH BACK OF THE BLASTOPORE LIPS. The former is in all 

 cases protogenetic, THE LATTER is DEUTEROGENETIC. 



The distinction between the two parts of the gut cavity is 

 quite distinctly seen in Axolotl and Hypogeophis, as Brachet's 

 figures of the former and Brauer's of the latter will demonstrate. 



In Rana it is a little more uncertain. I have often tried to 

 test this point by experiment in Rana temporaria and I think it 

 can be done but it is not easy. It might be done by inserting 

 fine hairs into the yolk cells just behind the dorsal lip of the 

 blastopore and then tracing them after growth in length has 

 continued for some time. 



It seems quite possible that the greater part of the gut cavity 

 of protogenetic origin is derived from the conversion of the 

 segmentation cavity into gastrula cavity by the movement of 



