270 



Mr. G. A. Drew on the 



mantle. The mid-gut, mg, and the ventral tube, vt, are both 

 shown in cross-section. 



This stage is of interest in showing the beginnings of tlie 

 ectodermal thickenings that form the pedal ganglia and the 

 ectodermal invaginations that form the otocysts. 



Fig. 3. 



Transverse section nf an embryo of iifty-eiLiht hours, taken just in front 

 of the second hand of cilia. my, mid-gut ; ."//, shell-jrland ; 

 i, test ; I't, ventral tube. 



As the shell-gland spreads out into the mantle, the embryo 

 becomes slightly compressed laterally, the foot appears, and 

 the visceral ganglia may be distinguished. 



Fig. 4 rejjresents a reconstructed embryo of one hundred 

 and five hours. The test-cells, <, are now very flat and much 

 thinner than at an earlier stage. The shell, .v, indicated in 

 outline, is well formed. The anterior adductor muscle, «<!, 

 occu}jies a rather isolated position. The posterior adductor 

 muscle, pa^ has just made its ajjpearance. The foot, /, is 

 pretty well formed, and has ruiniing over and united to its 



