POLYZOA. 



243 



connecting the month and anus in the adult. At the hinder 

 extremity of the blastopore there are present two conspicuously 

 large cells (fig. 128 B, me}, one on each side of the middle line. 

 These cells give rise to the mesoblast. On the completion of the 

 invagination the mesoblasts become covered by the epiblast (fig. 

 128 C, me). The blastopore then closes, but in the position it 

 occupied the epiblast becomes thickened to form the rudiment of 

 the vestibule, which at this stage constitutes a disc marked off by 

 a shallow groove from the remainder of the body. 



FIG. 128. THREE STAGES IN THE DEVELOPMENT OF PEDICELLINA ECHINATA. (After 

 Hatschek.) 



s.c. segmentation cavity; n.e. archenteron ; ep. epiblast; me. mesoblast; hy. 

 hypoblast. 



A is the commencing gastrula stage from the side in optical section. 



B is a slightly later stage from above in optical section. It shews the two primitive 

 mesoblast cells. 



C is a later stage after the closure of the blastopore, viewed from the side in optical 

 section. 



At the anterior extremity of this disc an invagination arises to 

 form the oesophagus (fig. 129 A, oe) ; and not long afterwards a 

 posterior invagination to form the rectum (fig. 129 B, an.t). The 



FIG. 129. Two STAGES IN THE DEVELOPMENT OF PEDICELLINA. (After Hatschek.) 



oe. oesophagus ; ae. archenteron ; an.i. anal invagination ; /. fold of epiblast ; 

 ,1.<j. ciliated disc ; x. problematical body derived from hypoblast (probably a bud). 



102 



