The Development of Ischnochiton. 585 



secondary trochoblast of Mead, a term which I shall adopt. The 

 remainiog cell 5 a ^-^ etc., m AmpJiiirite forms two cells one of which 

 also enters the functional prototroch. This latter feature does not 

 appear in Ischnochiton, though it will be seen that the history of this 

 cell is very intimately bound up with that of the prototroch. It enters 

 into the formation of a "supporting layer" of the velum, a third row 

 of cells lying below the two upper functional prototrochal rows i). I 

 am not certain that this lower layer becomes ciliated, as the move- 

 ments of the huge velar cilia prevent all observation in a living state. 

 If cilia be present they must be very delicate, and in preserved 

 specimens the killing agents give results too uncertain to be relied 

 upon. 



The following division forms the basal cell proper of the Molluscan 

 cross and the median cell (Figs. 16, 17). The spindle is perfectly 

 radial and all the divisions occur, with slight irregularities, at the 

 same time, and the products formed are similar in each quadrant, 

 there being now no pecularity of the posterior arm as in Crepidula. 

 At this stage therefore the cross is perfectly symmetrical with three 

 cells in each arm which lie in the antero-posterior axis and 90 <^ 

 removed. 



Each of the primary trochoblasts cleaves simultaneously giving 

 rise to four cells in each quadrant (Figs. 16, 17). The direction of 

 the cleavage is leiotropic and there is little subsequent shifting. This 

 is the last division that ever occurs in these cells. 



In all but the posterior quadrant the cells 2a^, etc. cleave in 

 a leiotropic direction to form a cell which like 2ai-2, etc. enters 

 into the formation of the supporting layer of the velum. These cells 

 are in very intimate relation with the secondary trochoblasts and at 

 the time of their formation give evidence of entering the velum, but 

 as the tip cells place themselves in their final position they become 

 situated below the prototroch forming a third row. 



6. Posterior Secondary Stomatoblast, 64 cells (Fig. 18). 



At this juncture a spindle arises in 2d2 and a cell forms in a 



leiotropic direction whose future history shows it to be a secondary 



stomatoblast. It is in intimate contact with the mesoblast (posterior 



1) Cells having the same origin and much the same relations arise 

 in Nereis, according to Wilson, and are designated the post-trochal 

 cells. They will receive attention in the consideration of the devel- 

 opment of the second quartette. 



38* 



