134 



INVERTEBRATA 



CHAP. 



stitutes the apical sense-organ, and into lq 112 , which form the rudi- 

 ment of the Annelidan cross, lq 112 bud off cells between them- 

 selves and the apical cells, and in this way the arms of the cross 

 are formed, that is to say, lq 112 divide into lq 1121 and lq 1122 . 



Turning now to the other cells of the upper hemisphere we find 

 that lq 12 behaves similarly. It divides into lq 121 and lq 122 , and 

 lq 121 further divides into lq 1211 and lq 1212 . These three cells in 

 each quadrant, lq 122 , lq 1212 , and lq 1211 , are in four curved series, and 

 this is also true of primitive Mollusca, but in higher Mollusca they 

 are arranged in four straight lines and form the upright cross 

 mentioned above. 



112 



,222 



If 21 I?' 



FIG. 102. Dorsal view of upper hemisphere of egg of Polygordius, in which seventy-six 

 cells have been formed. 



The rosette cells and the cells of the prototroch are left clear. The cells of the " Molluscan " cross 

 are cross-hatched. Those of the " Annelidan " cross are marked with circles. 



The group of cells lq 21 and lq 22 each divide into two sets of cells, 

 so that we have four daughters of lq 2 in each quadrant ;' and these 

 sixteen cells acquire long powerful cilia and constitute the prototroch. 

 They become large, clear, and vacuolated, and at first and this is 

 most interesting and important they form four discrete groups ; it 

 is only later that these groups coalesce to form a complete ring. 



If this description has been followed it will be seen that the upper 

 hemisphere of the egg, including the prototroch, when divisions 

 temporarily cease, consists of forty cells. In the 64-cell stage it 

 consists of course of thirty-two cells, since it forms exactly half 

 the egg, but the cells constituting the "Annelidan" and "Molluscan" 

 crosses divide once again, and this brings the total number of cells 

 up to forty. 



