DEVELOPMENT OF SPELERPES BILINEATUS 



191 



During this period, the cells at both sides of the blastopore have 

 been invaginating, until, dorsal to the lip, they are quite small, 

 but ventrally are still large. Although the movement of cells 

 into the blastopore continues ventrally it appears to cease above 

 the blastopore, which gradually shortens until it again becomes 

 a small crescent, (fig. 25), very similar to the crescent of earlier 

 stages. The difference in size of the cells bordering the blasto- 

 pore is the only feature by which this stage can be distinguished 

 from the similar earlier stage. Very soon the neural groove 



22 



24 



2 5 



26 



Figs. 22-275 



27 



appears which at once serves to orient the egg. It will be noted 

 that no ventral lip was formed. 



A slight rotation of the egg about a transverse horizontal axis 

 occurs at this time, bringing the blastopore up so that it lies just 

 beneath the equator. 



The blastopore now loses its crescentic shape by straightening 

 out into a horizontal groove except at its ends, which remain 

 bent downwards almost at right angles with the rest of the blasto- 

 pore (fig. 26). The ends soon shrink away, leaving the blasto- 



*Figs. 22 to 27 are diagrammatic. 



