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. 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 ; /. 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 



