258 C. W. METZ. 



dense body, which may be traced through all the stages. The 

 conclusion seems justified, therefore, that synapsis does not occur 

 in the looped regions of the three large chromosome pairs. 



THE " DENSE BODY." l 



This body, which appears to take the place of the nucleolus in 

 other flies, has such a consistent connection with one of the large 

 pairs of chromosomes throughout the. growth period that its struc- 

 ture and history may be considered in some detail. It is usually 

 ovoid or spherical, deeply staining in iron haematoxylin, and clear 

 cut in outline. In some cases it is clearly bipartite in structure, 

 and rarely it is divided into entirely separate parts. As already 

 noted, it is attached to both members of one of the large chromo- 

 some pairs, probably the J-shaped pair. When it is divided into 

 two parts, as in Figs. 17 to 19, this independent attachment is 

 shown clearly. Associated with the dense body, on the side oppo- 

 site the attachment to the chromosomes, is an achromatic structure 

 (presumably a plasmosome), of variable size and irregular outline, 

 as shown in Figs, n, 12, and 18. 



The origin of the dense body can not be traced accurately 

 enough to demonstrate that it arises directly and equally from the 

 two members of the one pair of chromosomes, but its subsequent 

 history (attachment, bipartite structure, behavior in late prophase) 

 suggests such an origin. 



During the prophase of the first spermatocyte division, as the 

 chromosomes go on the spindle, the dense body diminishes in size. 

 It gives the appearance of being taken up, in part at least, by the 

 attached chromosomes, for there is little indication of any of it 

 diffusing or breaking off. It separates into two components at 

 this time, coincident with the separation of the two chromosomes 

 (Figs. 20, 21 ) preparatory to the reduction division. They persist 

 in this condition up to, and perhaps through, the metaphase, as do 

 the somewhat similar " chromosome vesicles " observed by Caro- 

 thers ('13) in the Orthoptera Brachystola and Arphia. I have not 

 attempted to follow their history beyond this point. 



i Since the chemical nature of this body is not known it has been thought 

 better to apply a descriptive term to it, rather than to call it a nucleolus, 

 karyosome, or chromosome vesicle, each of which it resembles in certain 

 respects. 



