184 MONTGOMERY A STUDY OF THE CHROMOSOMES 



transverse constriction, about the size of a univalent component of one of the bivalent 

 chromosomes. This is the chromosome marked x in Fig. 157 ; in this figure is shown 

 also the chromatin nucleolus (N. 2), but only two of the five bivalent chromosomes. 

 This sixth small chromosome is univalent and unipartite, and evidently is the odd, 

 eleventh, chromosome of the spermatogonium, which had no fellow to combine with 

 during the synapsis. It is always recognizable on lateral views (Fig. 157) by its peculiar 

 volume and form, and even on pole views of the chromosomal plane is recognizable by 

 its lesser depth (Fig. 156, x). 



The first maturation division halves all seven elements (Fig. 158, anaphase), being 

 a transverse (reducing) division of the five bivalent chromosomes and of the chromatin 

 nucleolus, but in what plane the univalent chromosome (x) divides could not be deter- 

 mined on account of its nearly spherical form. Apparently also in the second matura- 

 tion division all the six chromosomes become divided, since in the spermatid six chroma- 

 tin elements can frequently be counted ; but I am not certain that the sixth chromosome 

 does become divided in this mitosis. 



The reduction in the number of chromosomes for this species is accordingly : 

 /Spermatogonium, two univalent chromatin nucleoli, eleven univalent chromosomes ; first 

 spermatocyte, one bivalent chromatin nucleolus, five bivalent chromosomes, one univalent 

 chromosome ; second spermatocyte, one univalent chromatin nucleolus, five univalent 

 chromosomes, one semivalent chromosome. 



27. Oncopeltus fasciatus Dall. 



Eight testes of this species were studied. 



The rest stage of the spermatogonium (PL IV, Fig. 159) shows usually one com- 

 paratively large, elongate chromatin nucleolus (N. 2), which is generally peripheral in 

 position ; this apparently represents two joined end to end, for sometimes two separate 

 ones can be seen. 



In the spermatogonic monaster there are sixteen chromatin elements (Fig. 160). 

 Fourteen are chromosomes and two are chromatin nucleoli, as the relations in the sperma- 

 tocyte mitoses will demonstrate. But it is difficult to determine which two are the 

 chromatin nucleoli, all sixteen elements being of approximately equal size, though, 

 judging by analogy with the other species of the family, they are probably the smallest 

 two (N. 2, Fig. 160). 



In the synapsis the fourteen chromosomes unite to form seven bivalent ones. But 

 there is never any very close union of the two chromatin nucleoli, and in the rest stage 

 of the spermatocytes the following conditions are found : (1) the chromatin nucleoli 

 apposed to one another and to the true nucleolus (Fig. 161) ; (2) apposed to one 



