372 



CUCULLANUS. 



grow, while the growth of the epiblastic layer continues. As a 

 consequence of this the sides of the plate begin to fold over 

 towards the side of the hypoblast (fig. 166 D.) This folding 

 results in the formation of a remarkably constituted gastrula, 

 which has the form of a hollow two-layered cylinder with an 

 incompletely closed slit on one side (fig. 166 E, bl.p]. This slit 

 has the value of a blastopore. It becomes closed by the coales- 

 cence of the two edges, a process which commences posteriorly, 



n 



FIG. 166. 



A. 

 B. 

 C. 

 D. 

 E. 

 F. 



VARIOUS STAGES IN THE DEVELOPMENT OF CUCULLANUS ELEGANS. 



(From Biitschli.) 



Surface view of flattened embryo at an early stage in the segmentation. 

 Side view of an embryo at a somewhat later stage, in optical section. 

 Flattened embryo at the completion of segmentation. 

 Embryo at the commencement of the gastrula stage. 

 Embryo when the blastopore is reduced to a mere slit. 



Vermiform embryo after the division of the alimentary tract into cesophageal 

 and glandular divisions. 



m. mouth ; ep. epiblast ; hy. hypoblast ; me. inesoblast ; ce. oesophagus ; bl.p. blas- 

 topore. 



and then gradually extends forwards. In front the blastopore 

 never becomes completely closed, but remains as the permanent 

 mouth. The embryo after these changes has a worm-like form, 

 which becomes the more obvious as it grows in length and 

 becomes curved (fig. 166 F). 



The hypoblast of the embryo gives rise to the alimentary 



