DEVELOPMENT OF SPELERPES BILINEATUS 



197 



migration of yolk-cells, disregarded in recent years, appears, 

 according to Moquin-Tandon ('76), to have been first noticed by 

 Strieker in a paper I have not seen. In fig. 31, the blastocoele 

 is filled with migrating cells, which cause the surface of the living 

 egg to appear mottled. The archenteron has become enlarged 

 but there has been little or no more invagination at the dorsal 



Figs. 32-35 



lip. Opposite it, a short cleft, c, has appeared while the exposed 

 surface of the yolk has decreased. Fig. 32 is drawn from an egg 

 of the same age as fig. 31. It is more developed in some parts 

 although less in others. 



Gastrulation, or better, invagination is nearly completed in the 

 egg shown in fig. 33. The ectoderm, reduced to a single layer of 

 cells above, is still several layers thick at the blastopore. The 



