56 TRACHYMEDUS^E. 



afterwards the hitherto uniform hypoblast becomes divided up into distinct 

 cells. The thin inner pellicle which persists after the rupture of the outer 

 membrane becomes in the meantime absorbed. With these changes the 

 embryo practically acquires the characters of the adult. 



TrachymedusaB. Amongst the Trachymedusae, which as 

 has now been satisfactorily established develop directly without 

 alternations of generations, the embryology of species both of 

 the Geryonidae and the ^Eginidae has been studied. 



In all the types so far investigated the hypoblast is formed 

 by delamination, and there is a more or less well-marked planula 

 stage. 



The development of Geryonia (Carmarina) hastata has been 

 studied by Fol (No. 155) and MetschnikofT (No. 163) 1 . The 

 ovum, when laid, is invested by a delicate vitelline membrane 

 and mucous covering. Its protoplasm is formed of an outer 

 granular and dense layer, and a central mass of a more spongy 

 character. The segmentation is complete and regular, and up 

 to the time when thirty-two segments 

 have appeared each segment is composed 

 of both constituents of the protoplasm 

 of the ovum. A segmentation cavity 

 appears when sixteen segments are 

 formed, and becomes somewhat larger at 

 the stage with thirty-two. At this stage 

 the process of delamination commences. 

 Each of the thirty-two segments, as 

 shewn in the accompanying diagram 



(fig. 7O), becomes divided into two unequal DELAMINATION OF THE 

 ' , r , . r , OVUM OF GERYONIA. 



parts. The smaller of these is formed (Copied from Fol.) 



almost entirely of granular material ; segmentation cavity ; 



. , . . r . . a. endoplasm ; b. ectoplasm. 



the larger contains portions of both T he dotted lines shew the 

 kinds of protoplasm. In the next seg- C ? u . r5 5 e of the next planes of 



division. 



mentation the thirty-two large cells only 



are concerned, and in each of these the line of division passes 

 between the granular and the transparent protoplasm. The 

 sixty-four lenticular masses of granular protoplasm thus formed 

 constitute an outer closed epiblastic vesicle, within which the 



1 In the succeeding account I have followed Fol, who differs in some nvnor points 

 from Metschnikoff. 



