78 EDMUND B. WILSON 



relation to the spindle-axis, often lying at right angles to the 

 latter. Apparently therefore its duality arises quite independ- 

 ently of the spindle or astral rays, and its constant position in 

 the fully formed spindle is the result of a later adjustment. In 

 this species, as in many others, each chromosome is connected with 

 the pole by a bundle of delicate fibers. In case of the d-chromo- 

 some this bundle is very broad, but I cannot be sure that it is 

 double. 



At first sight any observer would, I think, take the d-chromo- 

 some to be merely a result of the accidental superposition or close 

 adhesion of two separate dyads of unequal size; but such an inter- 

 pretation is inadmissible. When all the chromosomes can be 

 unmistakably seen, the d-chromosome is found to constitute one 

 of the seven separate elements invariably present in this division; 

 and since the diploid number is 14 in both sexes this chromosome 

 must represent one chromosome, not two, of the original sperma- 

 togonial groups. It is certain, therefore, that the double appear- 

 ance does not result from close apposition of two separate chromo- 

 somes; it is therefore not a " tetrad" in the ordinary sense of the 

 word i.e., not one that results from the synapsis of two chromo- 

 somes that are originally separate in the diploid groups. 



2. The first spermatocyte-division 



This division requires only brief mention. As stated, it shows 

 eight separate chromosomes, of which the only one that can be 

 positively identified is the Y-chromosome of N. viridula. This 

 chromosome, always immediately recognizable in this species 

 by its small size (3 c, d, f, g, i), figs. 12, 13), is usually central in 

 position, like the m-chromosome of the Coreidae, but this is not 

 invariable. Since it divides equally, and without association with 

 any other chromosome (3 g) it is evident that the two idiochro- 

 mosomes must be separate and univalent in this division. In N. 

 hilaris (3 a, b, figs. 10, 11) the eight chromosomes usually form an 

 irregular ring, there is no central chromosome, and neither idio- 

 chromosome can be certainly recognized. It nevertheless seems 

 a safe inference from what is seen in N. viridula that the two 

 idiochromosomes are here also separate and univalent. 



