40 The Chromatin in the Development of Hybrids 



was found that such sections might include the entire thickness of the 

 spindles in the earlier stages of cleavage. This was a matter of con- 

 siderable importance since it was desirable to get all of the chromosomes 

 of a given spindle in the same section. All the staining was done with 

 Haidenhain's hsematoxylin. This stain was found perfectly satisfactory, 

 find because of its simplicity and the ease with which it can be controlled 

 was used exclusively. 



3. Description of the Chromosomes. — The chromosomes of Fundulus 

 heteroclitus are long, slender and usually straight. They measure 3.18 

 micra in length. In a given anaphase all the chromosomes are of prac- 

 tically the same length. Just before breaking up into the chromosomal 

 vesicles the constituent chromomeres can commonly be made out. These 

 number four in nearly all cases that I have counted. In one instance 

 one of the chromosomes had five. The number of chromosomes is 36.^ 

 In their migrations to the poles they lie alongside of each other parallel, 

 for the most part, with the spindle fibres so that at the anaphase their 

 form can be easily made out (Fig. 11, Plate II). 



The chromosomes of Menidia notata are short and usually more or less 

 curved. They are sometimes straight and in some cases slightly sig- 

 moid. They measure 1.00 micron in length. As in Fundulus, they have 

 a uniform size in any given anaphase (Fig. 12, Plate II). I have tried 

 to make out the component chromomeres, but without success. The num- 

 ber of chromosomes is about 36. I have not been able to count them 

 definitely. 



In Figure 13 (Plate II), are given the two kinds of chromosomes 

 drawn to the same scale. Both the relative size and the difference in 

 form are shown. The difference in the two chromosomes comes out very 

 strikingly also when seen side by side in the same cell [Figs. 14, 15 

 (Plate II) and 29, 30 (Plate IV)]. The small chromosomes are here 

 grouped together on one side of each half of the spindle and the long 

 ones on the other side of the spindle. 



There can be no doubt that these two diverse forms of chromosomes 

 occurring in the hybrid eggs are the chromosomes of the two diverse par- 

 ents which, notwithstanding their association with strange chromosomes 

 in a strange cytoplasm, are evidently functioning in a perfectly normal 

 manner. A description of their behavior from the time of conjugation 

 to a late cleavage stage is the purpose of the present section. Before en- 



2 In only one instance was I able to count the number satisfactorily. Every 

 chromosome was definitely distinguished and counted in this case. I had, however, 

 concluded that 36 was the approximate number from numerous counts made on cross- 

 sections of anaphase spindles. 



