116 



SYCANDRA. 



amount of pigment near their inner ends (en.), and the other and 

 larger area of the thirty-two granular cells already mentioned (ec.). 

 Fifteen or sixteen of these are arranged as a special ring on the 



ec 



en 



en\ 



c.s. 



FlG. 65. TWO FREE STAGES IN THE DEVELOPMENT OP SYCANDRA BAPHANUS. (Copied 



from Schulze.) 



A. Amphiblastula stage. 



B. A later stage after the ciliated cells have commenced to become invaginated. 



cs. segmentation cavity; ec. granular cells which will form the epiblast; en. ciliated 

 cells which become invaginated to form the hypoblast. 



border of the clear cells. In the centre of the embryo is a seg- 

 mentation cavity (c.s.) which lies between the granular and the clear 

 cells, but is mainly bounded by the vaulted inner surface of the latter. 

 This stage is known as the amphiblastula stage. During the later 

 periods of the amphiblastula stage a cavity appears in the granular 

 cells dividing them into two layers. After the larva has for some 

 time enjoyed a free existence, a remarkable series of changes take 

 place, which result in the invagination of the half of it formed of the 

 clear cells, and form a prelude to the permanent attachment of the 

 larva. The entire process of invagination is completed in about half 

 an hour. The whole embryo first becomes flattened, but especially 

 the ciliated half, which gradually becomes less prominent (fig. 65 B) ; 

 and still later the cells composing it undergo a true process of 

 invagination. As a result of this invagination the segmentation 

 cavity is obliterated, and the larva assumes a compressed plano-convex 

 form, with a central gastrula cavity, and a blastopore in the middle of 

 the flattened surface. The two layers of the gastrula may now be 

 spoken of as epiblast and hypoblast. The blastopore becomes gradu- 

 ally narrowed by the growth over it of the outer row of granular cells. 

 When it has become very small the attachment of the larva takes 

 place by the flat surface where the blastopore is situated. It is 

 effected by protoplasmic processes of the outer ring of epiblast cells, 



