159 



the part between the ueurenteric canal and the anus and is post-anal, 

 and a very small pre-anal area (v. Robinson and Assheton, 23, Fig. 25). 

 But this ventral area though small is just as important morphologi- 



Bl- 



> Sable 



, Sable 



Bl-'' 



Sable 

 Fig. 3. Fig. 4. 



Fig. 3. Rana temporaria. The sable was inserted in the mid dorsal line after the 

 blastopoi'e had become considerably contracted. 



Fig. 4. Rana temporaria. The sable had been inserted very shortly before the 

 disappearance of the yolk plug. 



cally as the more bulky dorsal 

 mass. 



If LwoFF had recognised this 

 he would not have encountered 

 the difficulty of having to give 

 an "ectodermal" origin to the 

 notochord and mesoblast in Am- 

 phioxus. The dorsal wall is no 

 more ectoblastogenetic than the 

 lateral and ventral walls. 



If we take the tip of the tail 

 of an advanced embryo we find 

 there a growing point (compared 

 functionally by Shipley many 

 years ago in a paper on Petro- 

 myzon to the meristematic tissue 

 forming the growing point of the 

 stem of a dicotyledon) and this 

 mass of proliferating tissue is 



' Sable 



Anus 



Fig. 5. Rana temporaria. The sable 

 inserted through the sides during formation 

 of the blastopore. 



