PLATYELMINTHES. 197 



cavity. The blastosphere is converted by invagination into a 

 gastrula (fig. 91 A). The blastopore is soon carried relatively 



FIG. 91. THREE STAGES IN THE DEVELOPMENT OF LINEUS. (After Barrois.) 



A is a side view in optical section. 



B and C are two later stages from the ventral (oral) surface. 



ae. archenteron ; sc. segmentation cavity ; hy. hypoblast ; me. mesoblast ; ep. epi- 

 blast ; m. mouth ; st. stomach ; pr. d. prostomial disc ; po. d. metastomial disc ; pr. 

 proboscis. 



forwards by the elongation backwards of the archenteron, and, 

 according to Barrois, actually forms the mouth. Owing to the 

 elongation of the archenteric cavity the embryo assumes a bila- 

 teral form (fig. 92 A) in which the dorsal and ventral surfaces 

 can be distinguished, the mouth (m.) being situated on the 

 ventral surface. 



Immediately after the completion of the gastrula a remarkable 

 series of phenomena takes place. The embryo when viewed 

 from the ventral surface assumes a pentagonal form (fig. 91 B), 

 and four invaginations of the epiblast make their appearance on 

 the ventral surface (fig. 92 A), two in front of {pr. d.) and two 

 behind {po. d.) the mouth ; they result in the formation of four 

 thickened discs. These discs soon become separated from the 

 external skin, which closes in forming an unbroken layer over 

 them (fig. 91 C). The discs grow rapidly, and first the prosto- 

 mial pair and subsequently the metastomial fuse together, and 

 finally the whole four unite into a continuous ventral plate ; 

 analogous it would seem to the ventral plate of chsetopodan and 



