POLYZOA. 293 



nated in the normal manner (fig. 128 A), the blastopore becomes 

 narrowed to a slit with an antero-posterior direction, i.e. parallel 

 to the line connecting the mouth 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 ; a.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 primi- 

 tive 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.i). 

 The oral disc and the oesophagus are richly ciliated. The 

 oesophagus first, and afterwards the rectum unite with the 

 archenteron (fig. 130), the walls of which soon become differ- 

 entiated into a stomach and intestine, and on the upper wall 

 of the former the hepatic cells become especially conspicuous 

 (fig. 130). 



