44 The Chromatin in the Development of Hybrids 



the process of splitting, already extend their free ends for some distance 

 on each side of the equatorial plane. The other group is made up of the 

 short chromosomes which, also in division, appear as large granules 

 rather than elongated structures. 



In the anaphase [Pigs. 14, 15 (Plate II), 29, 30 (Plate IV)], two 

 , groups of chromosomes occupy each half of the spindle. Their form here 

 comes most distinctly to view. Careful examination of each group shows 

 that it comprises chromosomes of only one type, so that the chromatin 

 material has not become mingled during the fusion of the pronuclei. 

 The two groups do not occupy the same position with reference to the 

 pole or the equatorial plane, i. e., they are not equidistant. In other 

 words, the two kinds of chromosomes are not in the same stage of migra- 

 tion. This physiological difference is not so well marked in the Fun- 

 dulus hybrid where the small ones become the stragglers [Figs. 14 (Plate 

 TI) and 29 (Plate IV) ], but is very distinct in the Menidia hybrid [Figs. 

 15 (Plate II) and 30 (Plate IV)]. 



As the chromosomes become transformed into the resting nucleus each 

 is converted into a vesicle in a manner essentially similar to that de- 

 scribed for Crepidula by Conklin. In an early stage the two groups 

 of vesicles can be distinguished by the difference in the size of the vesi- 

 cles. These fuse at first into larger ones, giving rise to a lobed nucleus. 

 At this stage it is no longer possible to tell the two kinds of vesicles apart. 

 The fusion continues until a single well-rounded resting nucleus results, 

 with all traces of its double character lost. 



6. Second Cleavage. — Although all traces of the maternal and paternal 

 chromatins are lost in the resting nucleus of the two-cell stage there is 

 very little doubt that they have really remained spatially distinct. That 

 this is so, is shown by the fact that when the chromatin forms into the 

 chromosomes of the next cleavage the two kinds again appear, and in all 

 the spindles examined they were again bilaterally distributed on the 

 spindle. 



The kinds of chromosomes can, naturally, be best distinguished during 

 the anaphase, but even in the metaphase this may be done. In such a 

 metaphase of a Menidia hybrid, for instance, Fig. 17, where the long 

 chromosomes are the introduced ones, there may be seen on the one side 

 of the spindle the long ones with the ends of a part of the chromo- 

 somes already extending toward the poles, while on the other side the 

 short ones may be seen confined with characteristic strictness to the equa- 

 torial plane. Figures 18 (Plate II) and 31 (Plate IV) represent one of 

 the groups of the migrating chromosomes of an anaphase of a second 

 cleavage. Here the chromosomes come most distinctly to view. The 



