No. 3.] THE EGG OF VI RBI US ZOSTERICOLA. 745 



into which the ^<g% is divided, it will be seen, are arranged quite 

 differently from those of Types I and II. 



Fourth and Fifth Cleavage. 



In all three types of segmentation the blastomeres continue 

 to divide, always in a direction at right angles to their former 

 division, until there are thirty-two blastomeres. The nuclei 

 now occupy positions at the surface of the ^g%, all of the 

 spindles in the 16-32-cell stage taking a direction tangential to 

 the surface. 



■ Sixth Cleavage. 



In passing from thirty-two to sixty-four blastomeres there is 

 the first indication of invagination. The thirty-two spindles 

 form in the normal way, and twenty-eight of the blastomeres 

 are divided by cleavage-planes. In the four remaining Blasto- 

 meres no cleavage-planes appear, and these four cells soon 

 begin to invaginate (PI. XXXVII, Figs. 18, 19, and 20). In 

 the thirty-two and sixty-four-cell stages the cleavage-planes do 

 not extend to the center of the yolk, and the four nuclei which 

 sink below the surface are no longer separated by distinct cell- 

 walls. 



When we trace back the history of the invaginating cells of 

 the three types of segmentation we find them arising from 

 different parts of the Q.gg, and by entirely different methods of 

 segmentation. If we regard these four cells, then, as equiva- 

 lent cells in each of the three types, we must conclude that in 

 the Qg% of Virbius the prospective value of the cells is not at 

 all a " function of their position," but that the value of each 

 cell is determined, as early as the two-cell stage, by some 

 process of qualitative division of the nuclei. 



Brown University, March i, 1895. 



