230 N. M. Stevens 



several views of the unequal pair in different stages of longitudinal 

 splitting. 



It will be seen from the figures and description given, that I find 

 no evidence that the two halves of the bivalent chromosomes 

 remain folded together parallel with each other, as described by 

 Zweiger; but the rings and U's and V's all gradually straighten out 

 so that the univalent elements of the dumb-bell-shaped chromo- 

 somes stand end to end in the spindle, and the bivalents often 

 appear as typical tetrads. 



Figs. 34 and 35 are early anaphases showing separation of the 

 univalent components of both the unequal heterochromosome pair 

 and the equal pairs. In my material the anaphase stages (Figs. 34, 

 35, 36) usually show nothing like Zweiger's "accessorisches Chro- 

 mosom " (Zweiger, '06, Zool. Anz., Fig. 13 ; Jena Zeit., Fig. 25), but 

 occasionally one finds it (Figs. 37 and 38). Fig. 37 is from the 

 Eisenach material and Fig. 38 from a Helgoland preparation. The 

 former preparation consisted of a single pair of testes in which it 

 was possible to count the chromosomes in a number of spermato- 

 gonial plates: all had 24 (Figs. 2 and 3). There were also a large 

 number of first spermatocyte plates, containing 12 chromosomes 

 without exception. There was only one small cyst of second sper- 

 matocytes in metaphase; 12 chromosomes were counted in 24 cases, 

 13 in 2 and 11 in one. It is therefore evident that in this individ- 

 ual the lagging pair could not be an additional pair of accessory 

 chromosomes making 26 for the spermatogonial number (Zweiger's 

 explanation). Many pairs of daughter plates of the first sper- 

 matocytes were counted and 12 daughter chromosomes found in 

 every case (Figs. 39 and 40). In these two pairs of daughter 

 plates the heterochromosomes x^, and Xg were distinguishable. 



In the second spermatocytes the usual number of chromosomes 

 is 12 (Fig. 41 b),hwt in all testes in which this stage was at all abun- 

 dant, there were occasional i I's (Fig. 41, a) and 13's (Fig. 41, c-g) 

 in the same cysts with the 12's. Multipolar first spermatocyte 

 spindles are sometimes seen, but they are not frequent enough to 

 account for the irregular numbers in the second spermatocytes. 

 Then, too, the numbers are always 11, 12 and 13 and there is no 

 reason why multipolar mitoses should not give numbers both 



