300 T. H. MORGAN 



dition, but does so before condensation sets in again, prepara- 

 tory to the next division. In one case, figure 15, the X chro- 

 mosome failed to draw out of the smaller cell, and one end only 

 lies within the nucleus of the larger cell. This end has begun to 

 open out as the other chromosomes have already done. The 

 condition of the other cells in the cyst makes this interpretation 

 probable. 



During the resting stage, the X chromosome becomes con- 

 densed, as shown in plate 2, figures 14, 15, 16, 17. The chromo- 

 somes in the larger cells next begin to condense, preparatory to 

 the second division. The X chromosome is distinctly larger 

 than the other two; all three divide equally to form the sperma- 

 tids (plate 2, figs. 18, 19,20). 



The double nature of the X chromosome in the first spermato- 

 cyte division is so apparent that it invites speculation concern- 

 ing the nature of the division. Presumably the second divison 

 occurs in the plane of the split, but it is impossible to follow this 

 chromosome through its resting stage. Similar cases for the 

 single X chromosome are known, and I can but follow the usual 

 interpretation, viz. that we are dealing with a precocious divi- 

 sion. The tetrad formation that occurs in such forms as Asca- 

 ris is interpreted as a double division, one of which is precocious. 

 The rapidity with which the two reduction divisions take place, 

 often without an intervening resting stage, indicates that each 

 of the chromosomes, even though mated in pairs, has under- 

 gone the preparatory stages of division. Hence two divisions 

 are necessary to separate the four elements. But if this were 

 the whole of the matter it is not apparent why, in a case like this 

 one, the halves of the X chromosome do not separate from each 

 other at the first division. It is perhaps little more than an 

 evasion of the difficulty to suggest that the divisions in the X 

 chromosome are not sufficiently advanced, when overtaken by 

 the first division. 



Janssens has proposed a view of the necessity of the two re- 

 duction divisions, based on his observations of the chiasma type. 

 His studies of Batracoseps have shown that two of the four 

 threads of the tetrad sometimes break and reunite so that two 



