408 



EMBRYOLOGY 



and the bipartite vesicle separates off from the archenteron 

 (Fig. 192 C). Meanwhile the blastopore has closed. In 

 their further development the two arms of the vesicle grow 

 around the intestine. They come in contact with each other 

 behind [and dorsad of] the intestine, and there form the 

 mesentery, which extends from the intestine to the body- 

 wall [in a direction oblique to the sagittal plane]. The 

 fundament of the water- vascular system now first makes its 

 appearance in the vaso-peritoneal vesicle as an outfolding 

 of the left half of the vesicle (Fig. 191 B, H). Projecting 

 at first only a little beyond the wall of the vaso-peritoneal 



Fig. 192.— .4 to C, sections through larvffi of Asterina gihhoia (after Ludwig). Bl, 

 blastopore ; D, intestine ; Fp, vaso-peritoneal vesicle ; r and I, right and left 

 sides. 



vesicle, it soon gives rise to five lobes, thus producing the 

 earliest fundament of the five radial stems of the water- 

 vascular system. At about the same time, an invagination 

 of the ectoderm takes place on the dorsal side of thq larva 

 opposite the larval mouth ; it grows inwards, and opens into 

 the left half of the vesicle. This is the dorsal pore of the 

 larva, which therefore effects a communication of the out- 

 side world with enterocoele and hydrocoele, for the separa- 

 tion of the latter from the enteroccele does not take place 

 until later. 



