276 KANSAS UNIVERSITY SCIENCE BULLETIN. 



zoSlogicai- laboratory. 



cleoli. The ribbons were cut usually twelve micra thick, in 

 order to make it possible for a whole nucleus to be seen in one 

 section, thus avoiding the uncertainties arising in the counting 

 of chromosomes, etc., from the use of fragmentary nuclei. 



METAPHASE OF THE SPERMATOGONIA. 



For convenience, the chromosomes have been numbered ac- 

 cording to their size, from smallest to largest, the system 

 being based upon the relative sizes of the twelve chromosomes 

 in the first spermatocyte. In polar views of the spermato- 

 gonial metaphase twenty-three chromosomes can be very 

 clearly seen, and it is apparent, as Sutton and Montgomery 

 have pointed out, that they occur in pairs. They have there- 

 fore been numbered in duplicate according to similarity of 

 size. In doing this it is assumed that the largest two of the 

 twenty-three, the No. 12's, will conjugate and form the largest, 

 No. 12, of the twelve first spermatocyte chromosomes ; that the 

 next largest two, No. ll's, of the spermatogonia will form 

 the one next largest. No. 11, of the spermatocyte; and so on 

 for each pair. In this way the twenty-three chromosomes 

 of a spermatogonium may be grouped into a series of eleven 

 pairs and one odd unpaired member. The odd one is probably 

 the assessory chromosome, although we cannot be absolutely 

 sure here which one of the complex it is. This much, however, 

 can be said; there are three chromosomes, the two No. lO's 

 and No. 5 (figs. 1, 2, 4 and 6), which are of about the same 

 size. In synapsis two of these will unite, and, of course, one 

 of them must be left unpaired (Wilson, '05). This, then, is 

 thought to be the accessory. It corresponds in size with the 

 accessory of the first spermatocyte, where there can be no 

 doubt as to its identity. 



Six polar views were drawn, four of which, figures 1-4, are 

 from one animal and one apiece, figures 5 and 6, from two 

 other animals. Of the chromosomes in each cell there can be 

 distinguished three large pairs, the 12's, ll's and lO's (Mont- 

 gomery, '05) ; an accessory No. 5, which, as before said, is 

 about the size of the No. lO's ; three pairs of very small ones, 

 and five pairs ranging in size between the largest and the 

 smallest. Usually the No. 4's and 6's may be recognized, but 

 the 7's, 8's and 9's, being very much alike in size, can hardly 

 be told apart. In the drawings it is not possible in every case 

 to show the real length or size of the chromosomes, since they 



