NEW TYPES OF CHROMOSOME DISTRIBUTION. 135 



the contraction-phase (Plate i, A), there is present in addition 

 to the plasmosome, a deeply staining body, which proves to be 

 the first appearance of the differential chromosomes. In the 

 spireme stage (Plate i, .0), shortly after synapsis, the two bodies 

 are present, but the chromatic mass is more spherical. A little 

 later (Plate i, C), the chromatic body has become broken up 

 and a plasmosome has formed around it. The next stage (Plate 

 i, D) shows the two bodies in contact and beginning to fuse. 

 About this time the chromatic mass breaks up for the first time 

 into its constituent elements, the four differential chromosomes, 

 the larger of which is noticeably elongate (Plate i, E). The two 

 plasmosomes fuse completely to form a large more or less 

 spherical one (Plate i, F). Plate i, G, shows the typical ap- 

 pearance of the nucleus throughout the greater part of the growth 

 period. The larger differential chromosome shortens into a 

 somewhat spherical form. All of them lie embedded in the pale 

 plasmosome and not in contact with it as described by Mont- 

 gomery. Nor do any of them, at any time, show an appearance 

 of being bipartite and their later behavior in the maturation divi- 

 sions shows conclusively that all four are univalent. As we shall 

 see later, Montgomery's idea as to the relations in Sinea were in- 

 correct, and this accounts in part for the wrong interpretation 

 in Prio nidus. In the prophase of the first division, when the 

 chromatin is beginning to condense to form the chromosomes, 

 the plasmosome disappears and the differential chromosomes 

 remain. 



In Syromastes, Wilson ('09) describes the same numerical 

 difference in the male and female somatic cells as here described 

 for Prionidus. The difference in that case, however, is brought 

 about by two chromosomes which act as a unit and which divide 

 in only one division. 



Sinea diadema Fabr. 



Montgomery ('01 and '06) described in part the behavior of 

 the chromosomes in a species which he called " Sinea diadema," 

 but the relations which I find in this species are entirely dif- 

 ferent from those which he described. There is, therefore, but 

 little doubt that he studied a different species. This again em- 



