8 Edmund B. Wilson, 



heterotropic. A second pair (3, 3) may usually be distinguished 

 as the next largest, and a third pair (7, 7) as the smallest, though 

 this is not always obvious. This pair probably correspond 

 to the " m-chromosomes " of my preceding paper. The remain- 

 ing three pairs are of nearly equal size, though sometimes they 

 clearly show a progressively graded series as in Fig. I, </, e. In 

 synapsis the six paired chromosomes become coupled, as usual, 

 to form six corresponding bivalents, while the large chromosome 

 remains as an unpaired univalent. During the whole growth- 

 period of the spermatocytes this chromosome remains in a con- 

 densed spheroidal state, forming a very large chromosome- 

 nucleolus. In the prophases of the first division it again elongates 

 and divides longitudinally in this division. Each secondary 

 spermatocyte accordingly receives seven chromosomes. In the 

 second division six of these (the products of the bivalents) again 

 divide equally, while the seventh (the large chromosome) passes 

 undivided to one pole (Fig. I, a). One-half of the spermatid 

 nuclei accordingly receive six chromosomes, the other half seven, 

 the additional one being the large heterotropic chromosome 

 (Fig. i, b, c). 



In the female the chromosome-groups of the dividing oogonia 

 and follicle-cells appear with a clearness not inferior to that shown 

 in the spermatogonial groups (Fig. I, g-i). It is at once apparent 

 that in these groups there are two very large chromosomes, equal 

 in size, in place of the single one that appears in the male, while 

 the remaining chromosomes show the same relations as in the 

 male. There are accordingly fourteen chromosomes in all, which 

 may be equally paired off, two by two, and no chromosome is 

 without a mate of corresponding size. Since the largest two are 

 of the same relative size as the single heterotropic chromosome 

 of the male, it is quite clear that one of them must have been 

 derived from a spermatozoon containing this chromosome, while 

 the other is its maternal mate or homologue. 



I have not been able to follow by actual observation the phe- 

 nomena of reduction, maturation and fertilization in the egg; 

 but the data are sufficient to show, with a degree of probability 

 only short of certainty, what must be the history of the chromo- 



