A: 



s soon as the flask-shaped ectodermic invagination from 

 the left side of tlie larva has reaclied the liydrocoele, it 

 gradually changes into a rather expansive hollow which stånds 

 in free communication with the exteriör at the place where 

 the invagination began. According to MacBride, this opening 

 becomes completely closed in larvse of Echinus esculentus 

 at au age of about 21 days only. In Echinus niiliaris the 

 communication remains open for a long time, and I must 



a., primary amniotic cavity. e., ectoderm of the Pluteas. ed., ectodermic 

 disc. h., hydrocoele. o., exteriör opening. s., stone-canal. st., stomach of 

 the Plåtens, c, ciliated band. 



confess that I never found the pouch-like invagination com- 

 pletely closed. The hollow, which may properly be named 

 >:>the primary amniotic cavity», has thin walls except as regards 

 its floor which remains very thick. The floor, also called 

 oral disc by MacBride, which is destined to be transformed 

 into oesophagus, nervous ring etc, is at lirst rounded in shape 

 but subsequently acquires a pentagonal form. I prefer to 

 call it solely »ectodermic disc». The hydrocoele, being almost 



