THE CLEAVAGE AND THE FORMATION OF THE GEEM- LAYERS. 6 



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 first 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 



FIG. 1. A-D, sections through embryos of Chiton PoliisA, the blastula and gastrula 

 stages (after KOWALEVSKY). bl t blastopore ; m, 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 was 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.] 



