4i8 HOLMES. [Vol. XVI. 



of the Q.gg, come to lie behind the masses of cells which form 

 the rudiment of the cerebral ganglia. 



The cells composing the head vesicle are given in the follow- 

 ing table : 



Cells of the posterior arm of the cross < 



Posterior trochoblasts 



Cells of the lateral arms of the cross < 



1^1.2.1.2 

 1^1.2.2 



j^I.2.2 



The number of cells in the head vesicle is twelve, of which 

 nine belong to the first and three to the second quartette. The 

 area of the head vesicle, when it reaches its maximum size, is 

 fully equal to that of the rest of the embryonic body. The 

 cells composing it are very thin and transparent and of rela- 

 tively enormous size. All of the cells which make up the head 

 vesicle were present in the egg after the division of the cells 

 I^*■^ I<5'■^ etc., which occurs at about the 64-cell stage, and 

 only one cell which is destined to form a part of the structure, 

 viz., id^'""'^, undergoes division after this period. The cells, 

 indeed, become so exceedingly thin in later stages that it is 

 difficult to see how their division could be effected. What 

 becomes of these cells when the head vesicle disappears is 

 uncertain. 



The Cell Lineage of the Prototroch. 



It has been long known that the velum in the pulmonate gas- 

 teropods is a rudimentary structure. It was first noticed by 

 Vogt, and has been described since in various pulmonates by 

 Rabl, Fol, and Lankaster. The velar lobes, which are such a 

 characteristic feature in the so-called veliger stage in other mol- 

 lusks, are absent in the pulmonates, and all that can be said to 

 correspond to the velum is a double row of ciliated cells extend- 



