CHROMOSOMES OF ACHOLLA MULTISPINOSA. 1/7 



ment in this division, although the large one always lies on the 

 periphery. 



As a result of the equal division of all the chromosomes in the 

 first division, the metaphase plate of the second maturation division 

 shows i6 chromosomes. In this division, however, there is 

 always a definite arrangement. Ten of the i6 form a more or 

 less regular ring while the remaining six, two medium sized, 

 three small and the extra large one, are arranged in a hexad group 

 in the middle. Five members of the hexad group, the two 

 medium sized and the three smaller ones, lie in one plane, while 

 the sixth member, the large chromosome, lies either above or 

 below the five on the other side of the equatorial plane. Fig. 2, 

 F, is a view of the second division metaphase, slightly to one 

 side, showing the arrangement of the hexad group. Fig. 2, E, 

 shows the ten chromosomes in the ring and the five in the middle. 

 The large one could not be shown without displacing it. The 

 ten chromosomes in the ring divide equally while the members 

 of the hexad group do not divide, but five of them, the two 

 medium sized and the three small ones, pass to one pole and the 

 large one to the opposite pole. The anaphases showing this un- 

 equal distribution are shown in Fig. 2, 6^ and H. Two classes of 

 spermatozoa are thus produced, differing in that one class con- 

 tains 1 5 chromosomes, the other 1 1. Further, since the oogonial 

 number is 30 and the spermatogonial 26, the reduced number 

 of chromosomes in the egg must be 15 and the two classes of 

 spermatozoa must be respectively male and female producing. 



Spermatozoon 15 -f- egg 15 = 30 (9) 

 Spermatozoon 1 1 + ^gg 15 = 26 (c^) 



As stated in the previous account, the size relations of the 

 chromosomes serve as an aid in reaching the above conclusions. 

 If the fifteen-chromosome class of spermatozoa meets an &gg with 

 1 5 chromosomes, three of which are small, the offspring should 

 have 30 chromosomes, six of which should be small. This is 

 what we find in the female cells. If the eleven-chromsome class 

 of spermatozoa meets the same &gg, the cells of the embryo 

 should have 26 chromosomes, three of which should be small 

 and one extra large. This condition is fulfilled in the male cells. 



