POSITION OF THE SPONGIDA. 



66 3 



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. i, 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 ectoderm 

 and entoderm. The blastopore becomes gradually 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 ectoderm 

 cells, which, together with the other ectoderm cells, now become 

 amoeboid. At the same time they become clearer and permit a 

 view of the interior of the gastrula. Between the ectoderm cells 

 and the entoderm cells which line the gastrula cavity there arises 

 a hyaline structureless layer, which is more closely attached to 

 the ectoderm than to the entoderm, and is probably derived from 

 the former. A view of the gastrula stage after the larva has 

 become fixed is given in fig. 2. 



FIG. 2. 



ec 



Fixed Gastrula stage of Sycandra raphanus (copied from Schulze). 



The figure shews the amoeboid ectoderm cells (ec) derived from the granular cells of 

 the earlier stage, and the columnar entoderm 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. 



