GERMINAL LAYERS Of THE EMBRYO. 
251 
Eig. 4. — Two Stages in the Developmetit of Stephanomia pictum^ to illus- 
trate delamination. (After Metschnikoff.) A. Stage after the dela- 
mination ; ep. epiblastic invagination to form pneumatocyst. B. Later 
stage after the formation of the gastric cavity in the solid hypoblast. 
po. polypite ; t. tentacle ; pp. pneumatocyst ; ep. epiblast of pneuma- 
tocyst ; hy. hypoblast surrounding pneumatocyst. 
of a blastosphere in the cells of which the protoplasm is differ- 
entiated into an inner and an outer part. By a subsequent 
Fig. 5. — Three Ijarval Stages of Eucope poly sty la. (After Kowalevsky.) 
A. Blastosphere stage with hypoblast spheres becoming budded off 
into central cavity. B. Planula stage with solid hypoblast. C. Pla- 
nula siage wilh a gastric cavity, ep. epiblast; hy. hypoblast ; al. gas- 
tric cavity. 
process the inner parts of the cells become separated from 
