18 



ccrsidrrrtior. 



About the tirrr the blactula becore?. ciliated ard be- 

 gins to sv;ir, usually eight to ten hourn after f ertilir:atior. , 

 the cellr. at the ^'octerior end of the larva becore so-e- 

 what tal'er thar^ those i-^ the other regions; and fro'T^ 

 these cells relatively few in nunber, the endoderr^ arises. 

 The for'-atior of the endoderr in Stprot'oca is, in a general 

 way, siriilar to that dereribed by letschrilcff in his 

 "E- bryolcgische Studien an t-'edusen " for Clytia f l^'Vicl"!^., 

 Clytia virid leans and Cctorchis Gepexbauri . The endoder- 

 nal cellr are river^ off frcF he Icv-er end of the blastula 

 and are Dushed into the blastoccele. At first a single cell 

 ray be budce^' off. Gradually more cells are given off, 

 and those first set free divide; sr that byjthf crntinuation 

 of this nrocers for an indefinite ti^^e, the blastoccele be- 

 ccTes filled solidly fror the a-terior to the nostericr 

 end. Pictures If, 1£ and 20 are frcrr sections of blastulae 

 in Vi'hich thr fcrration of the endoderr ir ir different 



