A Study of Chromosomes and Chromatin Nucleoli in Euschistus crassus. 53 
stages of the growth period it is very rarely differentiated, we may 
say, that typically it does not appear until a rather late stage of the 
growth period, in this particular resembling the chromatic nucleoliis 
(secondary nucleolus) of Allolobophora foetida. Foot & Strobell('09)i). 
There is no evidence that this chromatin nucleolus in the germinal 
vesicle of Euschistus crassus contributes to the formation of the chroino- 
sonies, as the chromatic thread and the chromatic nucleolus seeni to 
develop concomitantly and not one at the expense of the other. Pho- 
tos 54 — 57. 
We have here a further example of -the inconstancy in the relation 
between the nucleolus and the chromosomes. We have undoubted evi- 
dence that in many forms the chromosomes are evolved from a part 
or all of a substance which at one phase of development appears as a 
nucleolus, whereas in other forms the two structures appear to be enth’ely 
independent. 
In Euschistus crassus we find this variability shown in the same 
form; in the spermatocytes four chromosomes arising from two chro- 
matic nucleoli and in the oöcytes the chromatic nucleolus and the chromo- 
somes appearing to be quite independent structures. It is difficult to 
harmonize such variability in the structural expressions of the cell with 
the theories that would seem to demand a rigid adherence to a definite 
mode of expression. 
Chromosomes. 
Photographs 21 — 35, plate III, demonstrate that the two spermato- 
cyte divisions of Euschistus crassus show the same phenomena as other 
Euschistus species. 
The behavior of the so-called sex determining pair of chromosomes 
(idiochromosomes) is typical. Fach idiochromosome dividing individually 
in the first division (photos 24 — 29) and the resulting two halves se- 
parating in the second division (photos 30 — 35). 
In determining the plane of division of the chromosomes, it is a 
question whether the form of the chromosome either at metaphase or 
prophase is a trust-worthy gidde. But as we seem to have no other 
morphological eine and this evidence is accepted when it does not clash 
In addition to Euschistus crassus we have found a similar cliromatin nucleolus 
in the growing oöcytes of a species of Liotropis which we have not yet had identified. 
In this form it is clearly differentiated in the very young oöcytes, before the 
beginning of the great growth period. 
