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, 



FIG. 166. 



A. 

 B. 

 C. 



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. 



D. Embryo at the commencement of the gastrula stage. 



E. Embryo when the blastopore is reduced to a mere slit. 



F. Vermiform embryo after the division of the alimentary tract into oesophageal 

 and glandular divisions. 



m. mouth; ep. epiblast; hy. hypoblast; me. mesoblast; a?, 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 



