Studies on Chromosomes 167 



of departure for the origin of the whole series of relations observed 

 in the genus. 



3 Individuals possessing twenty-three Chromosomes; one 

 Supernumerary 



This condition exists in all three species and has been found in 

 seven males and four females. In four of these males the super- 

 numerary is large (of approximately the same size as the small 

 idiochromosome, as in Fig. i, -/'); in three it is no larger than the 

 ra-chromosomes (as in Fig. I, /-/), and is indistinguishable from 

 the latter save in behavior. In each case, as already described, 

 the spermatogonia show 23 chromsomes and the first division 13; 

 and in those showing a small supernumerary in the first division 

 the spermatogonia always show three very small chromosomes. 



The grouping in the first division, though conforming to the 

 same general type, shows many variations of detail, as may be seen 

 from Fig 7, a-l, Photos 4-6. It is a curious fact that the form of 

 grouping is to some extent characteristic of the individual. For 

 example, the typical arrangement, with both idiochromosomes 

 and supernumerary outside the ring, is very common in Nos. 43 

 (Fig. i, /-/) and 20 (7, a-c), very rare in Nos. I, 2 (Fig. 7, /)and 

 49 (Fig. 7, f-h). In No. 49, very many of the first division meta- 

 phases show both supernumerary and small idiochromosome 

 lying inside the ring (Fig. 7, g-h). I am unable to suggest an 

 explanation of this. 



In this division all the chromosomes divide equally (Fig. 7, m-p), 

 so that each secondary spermatocyte receives 13 chromosomes. 

 The usual regrouping now takes place, and the idiochromosomes 

 couple as usual to form, an asymmetrical bivalent. The super- 

 numerary sometimes remains free (i. e., not attached to any other), 

 in which case 12 chromosomes appear in polar view (Fig. 8, b,d). 

 Much more frequently the supernumerary attaches itself to the 

 idiochromosome bivalent to form a triad element, polar views now 

 showing but II chromosomes (8, a, c\ one of which is compound. 

 The three components of such triads usually lie in a straight line, 

 the supernumerary being attached sometimes to the small idio- 



