A Study of Chromosomes and Chromatin Nucleoli in Euschistus crassus. 51 
I two relatively large chromatin nucleoli in the first spermatocyte, instead 
the one of niicleolus usually found in this group. In a former paper 
we gave the results of a count of 625 cells in a small area of a testis 
of Euschistus variolarius, 591 of these cells had one nucleolus, 27 had two 
nucleoli and 7 three nucleoü, and in the six species of Euschistus we have 
I studied, one chromatin nucleolus seems to be typical for the first sper- 
matocyte. The photographs of plate II demonstrate that two chromatin 
I nucleoli are distinctive of the fh’st spermatocytes of Euschistus crassus. 
, The photos of this plate further demonstrate that not only the so-called 
I sex chromosomes, but also a patr of the so-called “ordinary chromosomes” 
are evolved from a similar chromatic nucleolus. 
! The upper nucleolus in the cell, shown in photo 8 is apparently gi\üng 
I rise to the two idiochromosomes, for it is breaking down into two unequal 
parts. Photos 1, 2, 7 — 10 and 19 and 20 show that the two nucleoli present 
in this form are nearly equal in size, indicating that the second nucleolus 
i| probably gives rise to the pair of “ordinary chromosomes” most closely 
i related in size to the idiochromosomes. There are three nucleoh in 
photo 6, and photos 11 — 16, but in all these cases the two small ones 
are almost equal in size, and together are equivalent in size to the larger 
nucleolus, this indicating that each of the small nucleoli probably gives 
rise to one of the two “ordinary chromosomes”, which are as a rule 
developed from a single nucleolus. 
Photo 18 shows both the idiochromosomes and the pair of “ordinary 
chromosomes” already formed from the two chromatin nucleoli. The 
upper pair is evidently the two idiochromosomes, and the lower pair 
the “ordinary chromosomes”, the latter showing a transverse constric- 
tion probably indicating the first plane of division of this pair of “ordinary 
chromosomes”. This photograph shows that one member of this pair 
is larger than the other, but if this inequality in size is typical, it has 
not been possible to demonstrate it in later stages. Photo 17 shows a 
large nucleus with three nucleoli. It is impossible to determine whether 
the third nucleolus means that a second pair of “ordinary chromosomes” 
is to be developed from the third nucleolus or whether the presence of 
this third nucleolus merely represents superfluous chromatin. 
It is evident from the preparations of plate II that in these cells 
no additional nucleolus (achromatic plasmosome) is in evidence. Its 
absence in these photographs is as conspicuous as in those of other He- 
mipteran spermatocytes we have studied, and we may add that this is 
true for all of the Euschistus species we have thus far studied, but in 
view of the fact that other investigators of these forms insist that such 
4 * 
