DEVELOPMENT OF ECHINODERMS. 247 



Development of Echinoderms. 



The ovum undergoes total segmentation, and a hollow 

 ball of cells or blastosphere results. Apart from two alleged 

 cases of delamination, the gastrula is always formed by the 

 invagination pf this blastosphere. Ectoderm and endoderm, 

 or epiblast and hypoblast, are thus established. 



The mesoblast has a twofold origin : (a) from " mesen- 

 chyme " cells, which immigrate from the invaginated hypo- 

 blast into the segmentation cavity ; (b) by the outgrowing of 

 one or more ccelome pouches from the gastrula cavity or 

 archenteron. It is thus that the body cavity and the rudi- 

 ments of the water vascular system arise. 



According to Hertwig's fundamental , thesis this double 



FIG. 80. Stages in development of Echinoderms. 

 (After SELENKA.) 



i. Section of blastula of Synapta digitata (Holothuroid) with a 

 hint of gastrulation ; 2. Section of Gastrula of Toxopneustes brevi- 

 spinosus (sea urchin); ec., ectoderm; en., endoderm; m. y seg- 

 mentation cavity with mesenchyme cells in it ; 3. Section of larva 

 of Asterina gibbosa (starfish,) ; /., blastopore ; G., mesenteron ; 

 v.p., vaso-peritoneal vesicle ; r. and /., right and left sides. 



origin is a primitive condition, and the mesenchyme here, 

 as always, is non-epithelial and gives rise to the connective 

 tissues and to the vascular system. On the other hand, it 

 has been asserted that in Echinoderms .the mesenchyme is 

 not purely a " packing tissue," but may acquire a distinctly 

 epithelial character. Many of the early mesenchyme cells 

 are calciferous, combining to form the larva skeleton. 



