JOG 



INTRODUCTION. 



Many varieties are found in the animal kingdom of the types 



FIG. 58. Two STAGES IN THE DEVELOPMENT OF STEPHANOMIA PICTUM. 



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. pneumatophore ; ep. epiblastic invagination to form 

 pneumatocyst ; hy. hypoblast surrounding pnenmatocyst. 



of invagination and delamination just characterized, and in not a few 

 forms the layers originate in a manner which cannot be brought into 

 connection with either of these processes. 



^ 



FIG. 59. EPIBOLIC GASTEULA OF BONELLIA. (After Spengel). 



A. Stage when the four hypoblast cells are nearly enclosed. 



B. Stage after the formation of the mesoblast has commenced by an infolding of 

 the lips of the blastopore. 



ep, epiblast ; me. mesoblast ; bl. blastopore. 



The mesoblast usually originates subsequently to the two primary 

 layers. It then springs from one or both of the other layers, 

 but its modes of origin are so various that it would be useless 

 to attempt to classify them here. In cases of invagination it often arises 



