454 HOLOTHUROIDEA. 



the embryo becomes flattened, and the other more convex. On the 

 flattened side a fresh invagination arises, the opening of which forms 



FIG. 2-17. Two STAGES IN THE DEVELOPMENT OF HOLOTHURIA TCBULOSA VIEWED ix 



OPTICAL SECTION. (After Selenka.) 



A. Blastosphere stage at the close of segmentation. B. Gastrula stage. 



mr. micropyle; fl. choriou; s.c. segmentation cavity; bl. blastoderm; ep. epi- 

 blast; hy. hypoblast; ms. amoeboid cells derived from hypoblast; a.e. archenteron. 



the permanent mouth, the opening of the first invagination remain- 

 ing as the permanent anus (fig. 248 A). 



These changes give us the means of attaching definite names to 

 the various parts of the embryo. It deserves to be noted in the first 

 place that the embryo has assumed a distinctly bilateral form. There 

 is present a more or less concave surface extending from the mouth to 

 near the anus, which will be spoken of as the ventral surface. The 

 anus is situated at the posterior extremity. The convex surface 

 opposite the ventral surface forms the dorsal surface, which terminates 

 anteriorly in a rounded prse-oral prominence. 



It will be noticed in fig. 248 A that in addition to the primitive 

 anal invagination there is present a vesicle (v.p.). This vesicle is 

 directly formed by a constriction of the primitive archenteron (fig. 

 249 Vpv.), and is called by Selenka the vaso-peritoneal vesicle. It 

 gives origin to the epithelioid lining of the body cavity and water-vas- 

 cular system of the adult 1 . In the parts now developed we have the 

 rudiments of all the adult organs. 



The mouth and anal involutions (after the separation of the vaso- 

 peritoneal vesicle) meet and unite, a constriction indicating their 

 point of junction (fig. 248 B). Eventually the former gives rise to 



1 The origin of the vaso-peritoneal vesicle is not quite the same in all the species. 

 In Holothuria tubulosa it is separated from the csecal end of the archenteron ; the 

 remainder of which then grows towards the oral invagination. In Cucumaria the 

 archenteron forks (fig. 249); and one fork forms the vaso-peritoneal vesicle, and the 

 other the major part of the mesenteron. 



