34 The Chromatin in the Development of Hybrids 



through the two, four, eight, sixteen, etc., cells which in no way differ 

 from the corresponding stages of the normal eggs. The cleavage in the 

 hybrid eggs might show differences (1) in the irregularities in the size 

 of the cleavage cells or in the stages of different eggs in any given lot, 

 and (3) in the rate of cleavage. 



In a lot of Fundulus eggs, which have been taken from a single mother 

 fish and normally impregnated, all the eggs, except in rare instances, 

 will remain nearly perfectly abreast in their time of cleavage. I have 

 observed, however, that in a composite lot taken from a number of 

 females such perfect concert in the rate of cleavage may not obtain. 

 If the eggs are impregnated by sperm from a strange species even so 

 distantly related as Menidia this concert of cleavage is not affected so 

 far as I am able to detect. The same can be said for the reciprocal 

 cross. Three hybrid eggs came under my notice which should be men- 

 tioned in this connection. These had stopped in their development, 

 the one at the two-cell stage and the other two in the four-cell stage. 

 The blastomeres were in each case perfectly formed. The eggs were 

 all found in the same lot. The three abnormal ones were isolated to 

 watch their further fate. The eggs all died without dividing further. 

 I have endeavored to determine whether there was a greater irregu- 

 larity in the size of the blastomeres of the different cleavage stages, in 

 the hybrids than in the normals. My observations go to show that this 

 is not the case in the stages below the 32-cell stage. Beyond this I was 

 unable to make any comparison on account of the complexity of the 

 cell mass. In a lot of normal eggs of Fundulus there are always to be 

 foimd a number of .eggs in which there is more or less variation in the 

 size of sister blastomeres. One cell in the 2-cell stage, in an extreme 

 case, may be several times smaller than its mate. From this condition 

 to that of perfect equality in the size of the blastomeres there are all 

 intergradations. This inequality may be begun in the second or third 

 cleavage where, in addition, the cleavage planes may vary considerably 

 in their direction, giving rise to irregularities in the arrangement of the 

 blastomeres. Such irregularities everyone has noticed who has watched 

 any considerable number of cleaving fish eggs. In the hybrid eggs I 

 was unable to make out any difference in the extent to which such 

 irregularities occurred. This was certainly contrary to my expectations, 

 since I had considered it unlikely that two different chromatins of such 

 diverse origin would work so perfectly together through all the compli- 

 cated activities incident to cell division. That this may be, is evident 

 not only from the above consideration, but also from a closer study of 

 the internal phenomena described further on and from similar observa- 



