William J. Moenkhaiis 51 



one group to show one kind, and the other group, the other, but just this 

 I was unable to do satisfactorily. One of two things must be true, (1) 

 either the distinction between the two kinds of chromosomes had dis- 

 appeared or (2) they liad become mingled in the course of development. 

 The fact that I could make out two kinds of chromosomes which, how- 

 ever, were mingled upon the spindle, spoke directly for the latter view, 

 but I still hoped that a study of successive stages from the first cleavage 

 on might enable me to find conditions here similar to that found in other 

 forms. As already indicated, they found in Cyclops and Crepidula that 

 the maternal and paternal chromatin remained not only distinct but also 

 spatially separated up to varying late stages of development. Except 

 during the first and second and, in part, the third cleavage, this condi- 

 tion does not obtain in the hybrids under consideration. The chromo- 

 somes become mingled. This mingling probably has begun to a slight 

 extent in the second cleavage and is clearly well along in the third. By 

 the time cleavage is well along all the somatic cells have them mingled. 



The evidence that has been given to show that the two kinds of 

 chromatin remain spatially distinct in the forms referred to above is 

 very strong. I have shown beyond any doubt that this may be the case 

 for a very brief period in development. It is possible that the length of 

 this period may differ in different forms even to the extent that they 

 remain thus distinct throughout the entire embryonic period. It is 

 interesting in relation to this subject to compare the results of Riickert 

 on Cyclops strenuous and that of Hacker on Cyclops brevicornis. The 

 former found double nuclei, although in constantly decreasing number, 

 up to a late stage of development. The latter no longer found these 

 double nuclei in the 16- and 32-cell stage nor in the stage just preced- 

 ing the migration of the sex cells to the interior. He was, however, able 

 to distinguish the two masses by physiological differences in the sex 

 cells. This shows what a difference may obtain in two species of the 

 same genus. 



In the Menidia and Fundulus hybrids the bilateral arrangement of 

 the chromosomes is destroyed at about the same stage as in Cyclops brevi- 

 cornis, namely, at the third and fourth cleavage. According to Zoja 

 95, the two kinds of chromosomes in the Ascaris hybrid may be mingled 

 before the 12-cell stage. These observations suggest that possibly the 

 reason Hacker could not find the double nuclei beyond the 16-cell stage 

 lay in the fact that in Cyclops tenuicornis the chromosomes also became 

 mingled early in cleavage. Eiickert did not find any distinct grouping 

 of the chromosomes during the active phases of cell division beyond the 

 four- and eight-cell stage. In the light of Hacker's observations on a 



