134 



INVEETEBEATA 



CHAP. 



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

 ment of the Annelidan cross. Iq"^ 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, Iq"^ divide into Iq^^^^ and Iq^^z. 



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

 that lqi2 behaves similarly. It divides into Iq^^i and Iq^^^ and 

 lqi2i further divides into Iq^^n and Iq^^^^, These three cells in 

 each quadrant, Iq^^^ Iqi^is^ and Iq^^n, are in four curved series, and 

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

 are arranged in four straight lines and form the upright cross 

 mentioned above. 



Fig. 102. — Dor,sal view of upjper liemispliere of egg of PolygorcUus, in wliicii seventy-six 

 cells have been formed. 



The rosette cells and the cells of the prototroch ai-e left clear. The cells of the " MoUuscan " cross 

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



The group of cells Iq^^ and Iq^^ each divide into two sets of cells, 

 so that we have four daughters of Iq^ 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. 



