142 



SYCANDRA. 



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 border of the clear cells. In the centre of the 

 embryo is a segmentation 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 amphi- 

 blastula 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 promi- 

 nent (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 blasto- 

 pore in the middle 

 of the flattened sur- 

 face. The two layers 

 of the gastrula may 

 now be spoken of as 

 epiblast and hypo- 

 blast. The blasto- 

 pore becomes gradu- 

 ally narrowed by the 

 growth over it of the 

 outer row of granu- 

 lar 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, which, together with the other 



FIG. 66. FIXED GASTRULA STAGE OF SYCANDRA 

 RAPHANUS. (Copied from Schulze.) 



The figure shews the amoeboid epiblast cells (ec.) 

 derived from the granular cells of the earlier stage, and 

 the columnar hypoblast cells, lining the gastrula cavity, 

 derived from the ciliated cells of the earlier stage. The 

 larva is fixed by the amoeboid cells on the side on 

 which the blastopore is situated. 



