462 



INVEKTEBKATA 



CHAP. 



ectoderinic invagination situated at about one-third the length of the 

 larva from its front end. This invagination is of course the stomo- 

 daeum. Before the stomodaeum meets the gut, the latter becomes 

 divided by two constrictions into three regions. Of these regions the 

 hindermost is the intestine, the middle one becomes globular and 

 forms the stomach, whilst the most anterior forms the endodermal 

 portion of the oesophagus which meets and fuses with the stomodaeum 



B 



mes 



coe 



FIG. 356. Stages in the early development of Asterias nilgaris. (After Field.) 



A, blastula in optical longitudinal section. B, gastrula in optical longitudinal section showing the 

 formation of mesenchyme. C, older gastrula in optical longitudinal section. D, transverse section of 

 a larva slightly older than that represented in C, showing the formation of the coelom. ap, apical disc ; 

 coe, coelornic sac originating as pouches from the nrchenteron ; j/ies, mesenchyme ; rex, vesicle at the apex 

 of the archenteron. 



(Fig. 357, B), and thus completes the larval oesophagus. Along 

 the sides of this oesophagus a V-shaped band of strongly ciliated 

 epithelium is differentiated, which is termed the adoral ciliated 

 band (Fig. 357, C). It seems to be formed from both the ectodermal 

 and the endodermal region of the oesophagus. The angle of 

 the V is situated behind in the mid-ventral line. The limbs of 

 the V pass up the sides of the oesophagus, and their terminations are 



