ORIGIN OF THE GERMINAL LAYERS. 335 



that the gastrulae of these groups are simply modifications of the 

 normal type. 



2. Delamination. Three types of delamination may be 

 distinguished : 



a. Delamination where the cells of a solid morula become 

 divided into a superficial epiblast, and a central solid mass 

 in which the digestive cavity is subsequently hollowed out 

 (fig. 201). 



FlG. 201 i TWO STAGES IN THE DEVELOPMENT OF STEPHANOMIA PICTUM, 

 TO ILLUSTRATE THE FORMATION OF THE LAYERS BY DELAMINATION. (After 

 Metschnikoff.) 



A. Stage after the delamination; ep. epiblastic invagination to form pneuma- 

 tocyst. 



B. Later stage after the formation of the gastric cavity in the solid hypoblast. 

 po. polypite ; t. tentacle ; pp. pneumatocyst ; ep. epiblast of pneumatocyst ; hy. hypo- 

 blast surrounding pneumatocyst. 



b. Delamination where the segmented ovum has the form 

 of a blastosphere, the cells of which give rise by budding to 

 scattered cells in the interior of the vesicle, which, though they 

 may at first form a solid mass, finally arrange themselves in the 

 form of a definite layer around a central digestive cavity 

 (fig. 202). 



c. Delamination where the segmented ovum has the form 

 of a blastosphere in the cells of which the protoplasm is diffe- 

 rentiated into an inner and an outer part. By a subsequent 



