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 



row. 



Pigs. 57 and 58. —Sagittal section of two embryos of Palvdina oivipara (after T6n- 

 NIOES). ", anus; ent, entoderm; f, rudiment of foot; I, rudiment of liver; m, 

 mouth ; md, enteron ; mes, mesoderm-cells ; mf, lirst indications of the mantle-fold ; 

 s, shell-gland ; sf, shell-groove ; r, 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 



