MARINE TELEOSTEAN DEVELOPMENT. 



71 



and 18th April, 3rd and 4ith days. When segmen- 

 tation is complete the blastoderm undergoes a change of 

 a striking character. It becomes raised and separated from 

 the yolk so that a chamber, not coincident with the centre 

 of the disc, is formed between its under surface and the yolk 

 (Figs. 21 and 22, surface and lateral views). This chamber, 



FIG. 20 a. Edge of blastoderm with 

 nuclear zone (periblast). Highly 

 magnified. E. E. P. 



FIG. 21. Egg of Whiting with 



germinal cavity. Dorsal view. 



E. E. P. 



usually termed the segmentation-cavity, has also been styled 

 the 'germinal cavity' by Prof. Prince, to emphasise the fact 

 that it differs in some respects from the segmentation-cavity 

 of other types of vertebrate animals. 



The blastoderm at this stage shows no clear differentiation 

 into layers, though the upper stratum is usually distinguished 

 as a layer of ectoderm or epiblast-cells, and that below, forming 

 the main mass, as the lower-layer cells. 



A new series of changes now ensues. Formerly, in the 



FIG. 22. Egg of Whiting with many-celled blastoderm and germinal 

 cavity. Lateral view. E. E. P. 



