THE DEVELOPMENT OF PALUDINA. 



137 



the body. The shell-gland now becomes modified by the invagination 

 of its greatly thickened epithelium and by the appearance within the 

 invagination of the brown " chiton- plug " described by BUTSCHLI 

 (Fig. 57). During the further growth of the embryo, the gland 

 becomes flattened out and its cells lose their long columnar character 



me*. 



Fi<;s. 57 and 58. Sagittal section of two embryos of Pahvdina vivipara (after ToN- 

 NKiEs). a, anus; ent, entoderm ; /, rudiment of foot; I, rudiment of liver; m, 

 mouth ; aid, enteron ; mes, mesoderm-cells ; mf, first indications of the mantle-fold ; 

 s, shell-gland ; .>/', shell-groove; v, velum. 



and the epithelium finally becomes very thin (Fig. 58). At this 

 stage, lying above the shell-gland which is now slightly depressed, 

 there can be seen not only the remains of the chitinous plug but the 

 shell-integument itself (>'). The shell now extends rapidly over the 



