ECHINODERMATA 631 



are small ; the others larger and more yolky in proportion to the late- 

 ness of the stage at which they are set free. Fertilization takes place 

 in the sea or in brood pouches. Cleavage (radial, Fig. 196, i) is total 

 and forms a hollow, one-layered blastula (Fig. 438 A). This, by in- 

 vagination or unipolar ingrowth, forms a gastrula with a wide blasto- 

 coele into which typical mesenchyme cells wander from the wall of 



coe.- 





^-cil.bd. 



-stoni. 



\ 



/ 



"An. 



Fig. 438. Stages in the development of Asterias vulgaris. After Field. 

 A, Section of blastula. B, Section of gastrula. C, Section of older gastrula. 

 D, Three days' larva from the right-hand side. An. anus; arch, archenteron; 

 blc. blastocoele ; hip. blastopore ; cil.bd. ciliary band ; coe. rudiment of coelom ; 

 ect. ectoderm; ertd. endoderm; ent. enteron; mch. mesenchyme; mth. meso- 

 thelium; stom. stomodaeum. 



the archenteron. The blastopore becomes the anus, and the mouth is 

 formed by the breaking through of a stomodaeum. Meanwhile the 

 archenteron has budded off, at the anterior end, a vesicle which, by 

 processes that differ in detail in different cases, will give rise to the 

 three segments of the coelom described above (p. 627). The future 

 ventral side of the larva becomes concave. The larva is now known as 

 the Dipleurula. The cilia which uniformly covered the blastula be- 

 come sparse over most of the body but, except in the Crinoidea, grow 



