50 The Chromatin in the Development of Hybrids 



ity in their behavior during various phases of division. In the equa- 

 torial-plate stage the chromosomes of Menidia notata arrange them- 

 selves in an even band across the spindle. In a corresponding stage in 

 Fundulus, the plate presents a more ragged appearance, the ends of some 

 of the chromosomes extending out towards the pole. This difference is 

 especially well marked just at the time of splitting. In Figures 24 (Plate 

 III) and 33 (Plate IV) is shown a cell in later anaphase of Menidia 

 notata taken from about the middle cleavage stage of the embryo. The 

 chromosomes are in a compact group without any stragglers along the 

 spindle as is so common in the hybrid cells. Figure 25 (Plate III) is 

 taken from an early cleavage stage of Fundulus heteroclitus. All the 

 chromosomes are of the characteristic long form. In the later anaphase, 

 [Figure 26 (Plate III)], corresponding approximately to the stages 

 above given for Menidia notata (Figs. 24, 33), these chromosomes retain 

 their characteristic length and extend along the spindle for some distance. 

 If with these conditions in the normals corresponding stages in the 

 hybrid are compared (Figs. 22 and 32), it will be seen that in the latter 

 the conditions characteristic of both species are present. Here, as already 

 stated, the short chromosomes appear as a band nearer the pole, extending 

 across the spindle, and the longer ones, belonging to Fimdulus, extend 

 further along the spindle toward the equator. This somewhat tardy 

 migration of the longer chromosomes may be caused by their not being in 

 their native cytoplasm, for in the reciprocal cross where the conditions 

 are reversed this difference in the rate of migration does not obtain. 



11. Comparison with Other Forms. — "WHien I first discovered that 

 Menidia and Fundulus possessed two kinds of chromosomes and that 

 these could be distinguished in the first cleavage spindle, I went at once 

 to later cleavage stages for the further study of their behavior. In such 

 stages I could easily get great numbers of cells in all stages of division 

 in a single section. Those who had worked upon other forms had found 

 that even in late cleavage stages the double nuclei, representing the 

 maternal and paternal chromosomes, were more or less abundantly pres- 

 ent. In sections of such stages I found no difficulty in finding evidences 

 of the kind that had been employed by others in their studies upon other 

 forms, namely, the grouping of chromosomes into two groups, during 

 various stages of the division of the cell, and bilobed and double resting 

 nuclei and the rather constant presence of double nucleoli in each 

 nucleus (Fig. 27, Plate III). Knowing that the cells contained chromo- 

 somes of such different form as I had seen in the first cleavage, I was much 

 disappointed at my inability to identify the two kinds here. In those cells 

 where the chromosomes were grouped I had every reason to expect the 



