18 



EMBRYOLOGY 



After the completion of the swarming stage, and shortly 

 before the attachment of the larva, a shortening in the 

 direction of the chief axis takes place, which is accomplished 

 principally by a flattening of the flagellate, formerly bulging 

 cell-layer ; and an invagination of this layer quickly follows 

 the flattening, the result being that the cleavage cavity is 

 entirely obliterated. In this way a cap- shaped gastrula-stage 

 (Fig. 3 B) is reached. The outer granular layer of cells can 

 henceforth be considered as the ectoderm, and a circle of 



Fig. 5a.— Young, mortar-shaped Olynthus stage of Sycandra raphanus (after 

 F. E. Schulze). Os, osculum ; po, lateral inhalent pores of the body-wall. 



about sixteen of these cells, which are particularly prominent 

 and are known as marginal cells {Randzellen)^ surrounds the 

 wide gastrula mouth or blastopore, while the invaginated 

 flagellate layer represents the entoderm. 



The attachment of the larva now takes place by the 

 fixation of the edge of the gastrula mouth by means of 

 pseudopodia-like processes of the marginal cells to some 



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