144 ^- ^- Newman 



ous and not entirely surmountable for reasons that will soon 

 become clear. 



A large percentage of the eggs show intermediate stages between 

 the 2- and the 4-cell condition, and in many cases the first two 

 blastomeres undergo the second cleavage at different rates, pro- 

 ducing 3-cell conditions. In earlier studies all eggs were counted 

 as 4-cell stages if the second cleavage had begun, thus grouping 

 into one class all stages from the end of the 2-cell to the beginning 

 of the 8-cell condition. In practically all of the earlier experi- 

 ments the additional error was made ot allowing the development 

 to proceed a little too far, so that nearly all of the eggs had at 

 least begun the second cleavage. In later stages (8-, 16-, and 32- 

 cell conditions) it became a matter of great difficulty to assign the 

 various individuals to one of two classes, for the reason that 

 cleavage is far from regular either in time rate or the arrange- 

 ment of the cells. 



In all experiments described in the previous paper, where the 

 eggs of Fundulus majalis w^ere involved, the results show a slightly 

 more rapid rate of cleavage in the hybrid than in the pure-bred 

 eggs. The eggs of F. majalis were found to be much more suitable 

 for a statistical treatment of the rate of cleavage than those of 

 Fundulus heteroclitus because of their larger size, the greater 

 contrast between the color of the yolk and of the blastomeres, 

 and also, a matter of extreme importance, the fact that the eggs 

 if fertilized in a small amount of water, remain entirely separate 

 from one another and can be kept evenly distributed in a single 

 layer on the bottom of large flat dishes. All eggs can thus be 

 kept under identical conditions, so far as illumination, tempera- 

 ture, and oxygen supply are concerned. The eggs, moreover, are 

 not sticky and can readily be handled with needles or pipette. 

 Those of F. heteroclitus, on the other hand, invariably cohere in 

 clumps so that the innermost eggs are more or less cut off from 

 light and oxygen. It is possible, of course, to separate these 

 clumps, but the operation is a slow and tedious one and before it 

 can be completed many eggs will have been hindered in their 

 development. Even after these eggs have been separated they 

 are difficult to handle as they remain somewhat sticky for a long 



