THE CELL ORIGIN OF THE PROTOTROCH. 117 



After the next cleavage the i6-cell stage is formed, in which 

 the cells are disposed in four quartettes. Fig. 7 represents the 

 Q^^ in this stage seen from the side, but slightly rolled to 

 show all the eight cells of the 

 anterior hemisphere. The 

 polar globules lie in the mid- 

 dle of the hemisphere. The 

 derivation of the cells from 

 those of the 8-cell stage is in- 

 dicated by the arrows, and the 

 furrow between each pair of 

 cells has the general direction 

 of the loxodromic curve in 

 Fi"-. ■\. The dotted line in- 

 dicates the boundary between 

 the two hemispheres, and the 

 heavy lines separate the quad- 

 rants. The four cells which 

 are stippled in Fig. 7, one in 



each quadrant, are the pri- 

 mary trocJioblasts in the strict 



sense, for they contain all the 



material of the primary pro- 



totrocJi and no other material. 

 By the division of the six- 

 teen cells represented in Fig. 



7 the 32-cell stage is attained 



(Fig. 8). It consists of eight 



quartettes (four cells in each 



quadrant), two of them, the 



third and fourth, from the 



animal pole, being derivatives 



of the primary trochoblast 



and stippled as in Fig. 7. 



(Fig. 8 really represents the 



beg-inning of the transition between the 32- and 64-cell 



stage. Two of the cells of the subumbrella hemisphere have 



divided. These cells, however, do not concern us especially.) 



Fig. T. — Amphitrite, from left side and top, i6-cell 

 stage. Heavy solid lines divide the quadrants; 

 dotted lines divide the anterior and posterior 

 hemispheres. 



Fig. Z.^ Ainphitrite, left side, 32-cell stage. 



