Studies on Chromosomes 21 



species taken place in the female. The facts conclusively show 

 that such is not the case. 



The spermatogonial groups (Fig. 5, /) show fourteen chromo- 

 somes, all of which may be symmetrically paired. The smallest 

 pair, /', /, (as I showed in my first paper) are the idiochromosomes 

 as is shown by their characteristic behavior during the growth- 

 period and in the maturation-divisions. In synapsis the twelve 

 larger chromosomes couple to form six bivalents, while the idio- 

 chromosomes divide as separate univalents in the first spermat- 

 ocyte-division. Their products then conjugate as usual to form 

 the idiochromosome-dyad, which differs from all the forms hitherto 

 observed in being composed of two equal members. All the 

 spermatid-nuclei are accordingly exactly similar in appearance 

 and no visible dimorphism exists (cf. Fig. 4 of my first paper, 

 Wilson, '05, i). We should accordingly expect to find the 

 oogonial groups exactly similar to the spermatogonial; and such 

 is clearly shown to be the fact by the preparations, the oogonial 

 groups showing fourteen equally paired chromosomes among 

 which the idiochromosomes are readily recognizable by their 

 small size (Fig. 5> m)- 



In this case, therefore, alone among all those examined, no 

 visible differences are shown by the nuclei of the two sexes. 

 One pair of the chromosomes are, however, different in nature 

 from the others, as is shown by their different behavior in the 

 male in the growth-period and in synapsis; and it is quite clear 

 that the two members of this pair are always assigned to different 

 spermatozoa. In respect to this chromosome, therefore, the 

 spermatozoa fall into two classes as truly as the other forms, 

 though they cannot be distinguished by the eye. It is hardly 

 necessary to point out how important this case is in giving a firm 

 basis of comparison with the more usual forms in which, if we can 

 trust the existing accounts, all of the functional spermatozoa are 

 exactly alike in appearance, and no sexual differences of the chro- 

 mosome-groups are apparent. 



