ECHINODERMATA. 545 



eventually form the muscular system and the connective tissue. 

 These cells very probably have a bilaterally symmetrical origin. 

 This stage represents the gastrula stage which is common to all 

 Echinoderms. The invaginated sack is the archenteron. As it 

 grows larger one side of the embryo becomes flattened, and the 

 other more convex. On the flattened side a fresh invagination 



FlG. 247. TWO STAGES IN THE DEVELOPMENT OF HOLOTHURIA TUBULOSA 



VIEWED IN OPTICAL SECTION. (After Selenkn.) 



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

 mr. micropyle ; fl. chorion; s.c. segmentation cavity; bl. blastoderm; ep. epiblast; 

 Ay. hypoblast; tns. amoeboid cells derived from hypoblast ; a.c. archenteron. 



arises, the opening of which forms the permanent mouth, the 

 opening of the first invagination remaining 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 ex- 

 tending 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 prae-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 



B. II. 35 



