56 
Katharine Foot and E. C. Strobell 
chromosomes is the carrier of the sex determinant it must bc tlie Y chromo- 
some for the male and the X for the female, as Y is the only chromosome 
that is in both the so-called male producing spermatozoa and X is the only 
chromosome that is in both the so-called female producing spermatozoa. 
Tf in consideration of the fact that in many forms there is no Y 
chromosome, we place the male determinant in one of the ordinary chro- 
mosomes, this encounters the serious objection that no one ordinary 
chromosome can be in both the so-called male producing spermatids. 
If we assunie, for example, that it is in chromosome A, it can be in only 
one of the two male producing spermatids while the other A chromo- 
some is in one of the two female producing spermatids. If in order to 
insure the presence of the male determinant in both male producing sper- 
matids, we assume it is in both chromosomes of a pair (in A and B for 
example) then a male determinant is not only in both so-called male 
producing spermatozoa but also in both so-called female producing 
spermatozoa. 
We escape some of these difficulties in the many forms in which 
the X chromosome fails to divide in the first division instead of in the 
second, for in these cases the spermatids of one sex have like ordinary 
chromosomes and the male sex determinant can be hypothetically placed 
in any one of the ordinary chromosomes, but it must be further assumed 
that this male chromosome must never go to the same pole with the X 
(female) chromosome. 
The fact that in some closely related forms the four spermatids re- 
sulting from the two divisions differ so radically in their relation to the 
ordinary chromosomes challenges the hypotheses which claim a causal 
significance for even slight variations in such relations. 
Photos 36 — 53 demonstrate that twelve chromosomes is the number 
typical for Euschistus crassus. Three spermatogonial groups are shown 
in photos 36 — 38. Two ovarian groups in photos 44 and 45. Five, pre- 
sumably male embryo groups in photos 39 — 43, and seven female 
embryo groups in photos 47 — 53. A critical examination of the chromo- 
somes of these groups will show what we have demonstrated in several 
other forms, that the relative size of the individual chromosomes is by 
no means constant, “Making size relations of a chromosome a most 
uncertain guide for identification unlcss the difference in size is so ex- 
treme it allows for individual Variation”. Foot and Strobell '07. 
The largest bivalent of Euschistus crassus can be identified at a 
glance though its relative size may be very different, compare for 
example, photos 22 and 23. The next in size can be frequently iden- 
