412 



EMBRYOLOGY 



part of the posterior vesicle. Moreover, the anterior vesicle 

 has also altered in shape, for it divides into an extensive 

 saccular and a more narrow, canal-shaped portion (Fig. 196 

 B). The fundaments of the most important parts are thus 

 pi'oduced. The posterior bipartite vesicle constitutes the 

 fundament of the enterocoeles. It first separates from the 

 other parts, and then divides into a right and left coelomic 

 sac. Of the two processes from the neck between the anterior 

 and posterior vesicles, only the larger, dorsal one is said 

 to be retained, to constitute the fundament of the intestine, 

 whereas the smaller, ventral one disappears (Barrois). The 

 anterior vesicle, the hydrocoele, which already exhibits the 



mes~. 



^ 7 ~ -eat 



Fig. 19G.— ^ and B, embryos of Aide.don rosacea in optical section ; rteveloi^ment 

 of the enterocoele (ent) and hydroccele {h) (after J. Bakbois). d, intestine; mes, 

 cells of the mesenchyma ; st, stone canal. 



separation into water-vascular vesicle and stone canal, 

 remains united with the intestine for some time (Fig. 196 

 B). Later the hydroccele also separates from the intestine, 

 and then, as in other Echinoderm larvoB, the two enterocoeles 

 and the hydrocoele are found lying next to the intestine, 

 which has now considerably enlarged. However, the oral 

 and anal openings, as well as the water-vascular pore, are 

 still lacking. 



A vaso-peritoneal vesicle can scarcely be spoken of in 

 Antedon, unless the condition in which the already paired 

 enterocoeles are still connected with the hydrocoele by means 



