THE CLEAVAGE AND THE FORMATION OF THE GERM-LAYERS. d 



expression of a kind of radial symmetry, which is still more marked 

 in the Gastropoda at a somewhat later stage (Fig. 40 D and E), and, 

 according to Kowalevsky's figures, is sometimes also met with in 

 Chiton. At fii'st the cells increase in number in a more regular 

 manner than in the later stages. 



Cleavage results in the production of a somewhat flattened, and 

 therefore hemispherical blastula, the vegetative pole of which consists 

 of comparatively few but very large cells (Fig. 1 A.) As the cells 



a. 



c. 



£> 



Fig. 1. — A-D, sections through embryos of Chiton Polii at the blastula and gastrula 

 stages (after Kowalevsky). bl, blastopore ; »i, rudiment of the mesoderm ; w, rudi- 

 ment of the ciliated ring [velum]. 



continue to divide, an invagination of the vegetative half (B) takes 

 place. In this way the cleavage-cavity, which w r as never large, is 

 further compressed.* The invagination-gastrula (B) which at first is 

 somewhat depressed, now elongates in the direction of the invagina- 

 tion (C). The archenteron also grows larger. In its wall, near the 

 blastopore, there appear two cells which, as compared with the rest, 



* [Metcalf (No. I.) finds a large blastocoele which is not wholly obliterated 

 during the later development. — Ed.] 



