46 



LILLIE. 



[Vol. X. 



What determines the direction of the spindle and hence of the 

 cleavage ? Here again we cannot get back of the organization of 

 the cell. No mechanical explanation will suffice. Let us look 

 for a moment at the cleavages of X : The first position of the 

 spindle is on its left side ; the second position on the right side 

 (Figs. 25 and 27) ; the third in the 

 middle line towards the apical pole 

 (Figs. 33-35); the fourth in the 

 middle line towards the vegetative 

 pole (Fig. 39). (See accompany- 

 ing diagram.) In none of these 

 cases does the spindle occupy more 

 than a fraction of the diameter of 

 the blastomere in question. The 

 nucleus has been wandering 

 through the cytoplasm from one 

 side to the other, from the front 

 to the back, stopping at various 

 stations, and giving off a cell at 

 each one. Finally the nucleus stops in the centre of the cell 

 and a perfectly bilateral spindle (the fifth) is formed (Fig. 44). 

 Why docs it stop there.? Is it because its environment has 

 changed ? If so, the change is such as to elude the closest 

 scrutiny. In fact the cell is a builder which lays one stone 

 here, another there, each of which is placed with reference to 

 future development. 



- Diagram of the Cleavages of the 

 First Somatoblast X. 



III. Gastrula to Glochidium. 



(a) Gastrula to Young Larva. 



I. Ectodermal Organs. 



The formation of the shell-gland has already been described. 



Immediately afterwards the lumen is filled with a transparent, 



refringent substance, the first indication of the cuticular shell. 



A rapid evagination of the capacious gland now ensues. At 



the same time there is a thinning out, which proceeds more 



rapidly in the centre than at the sides. This is, of course, due 



to the extension of the area covered by the same cells. When 



