14 



Growth in length of the Vertebrate Embryo 



cells, Fig. 26 A. Brauer seems to draw a sharp distinction 

 between "animal cells" and "vegetative cells." I cannot see the 

 importance of so doing. All his figures show that the superficial 

 layer of the whole egg very quickly becomes segmented or at 

 any rate it becomes covered with a layer of cells. Possibly this 

 may be to a certain extent due to a sliding of the animal cells 

 over the surface, but it is far more probably due to a progressive 

 differentiation,, as it is in the frog. 



G 



Fig. 8. 



A, B, C, D, gastrulation stages in Amphioxus. After MacBride, 1910. E, F, G, 

 corresponding stages in Rana temporaria ; the white parts represent proto- 

 genetic ectoderm ; the black protogenetic endoderm, and the thick and thin 

 lines deuterogenetic endoderm and ectoderm respectively; sg segmentation 

 cavity. 



However that may be, a stage is eventually reached when 

 there is a circular blastopore established, with lips continuous 

 all round and with a superficial layer of cells on the outside and 

 an inner cell mass, Fig. 27, p. 49. 



In this stage the cavities which represent the segmentation 

 cavity, become confluent with a space caused by splitting between 

 the cells that radiate inwards from the blastopore lip. Thus in 

 Hypogeophis we have at this stage the future gut cavity well 

 defined into two regions : that which was in existence actually 

 before or at the time of the beginning of the blastoporic ingrowth, 

 and that which has been produced in connection with the growth 

 of that edge. 



