SEX DETERMINATION IN THE WHITE-FLY Dis 
practically the same stage of development, so that they can be 
easily distinguished from the cells of neighboring groups, which 
are generally at different stages. In addition, all except the 
earlier spermatogonia, clearly show cyst walls. Since the cell 
number is practically identical in all cysts which are at the same 
stage of development, it is possible to count the cells after each 
division, and thus determine whether a certain percentage is 
lost. If the number of spermatozoa is proportional to the number 
of spermatogonial cells, then it can justly be concluded that no 
abortive division has occurred and that all divisions have resulted 
in functional cells. That such is indeed the case seems to be 
pointed out by the following: 
Spermatogonial cysts show 7 to 8, 13 to 16, and 28 to 33 cells. 
Cysts with cells after the first of the last two divisions show 
52 to 59 cells. 
Cysts with cells after the final division or spermatids show 103 
Godel. 
Cysts w th ripe spermatozoa (which are easily counted when 
the sperm bundle is cut at right angles) show 101 to 123. 
These figures show, as is to be expected, that a few cells are lost 
during development. But, certainly, there ‘s no such essential 
loss as in case of the ant Camponotus (Lams, ’08), where each 
spermatocyte produces but two functional spermatozoa, or the 
bee, where only one is finally produced. The objection may be 
made that not all of the spermatozoa of T. vaporariorum may be 
functional, and a large percentage may be rudimentary—assum- 
ing that the numerical proof given is the only evidence available. 
The only answer that can be made here is that spermatids and 
spermatozoa, respectively, show no differences of size or structure. 
Comparing the phenomena here described with those seen in 
the nearly related groups of Aphids and Phylloxera, it will be 
seen that in those insects the chromosomes of the somatic cells 
have a very characteristic oblong or else lengthened shape. With 
a few exceptions, this form is lost in the maturation divisions, 
where the chromosomes become rounded and lumplike. ‘This is 
true of both sexes. Exactly the same feature is noticed in the 
female of T. vaporariorum, the chromosomes of which seem to 
JOURNAL OF MORPHOLOGY, VOL. 34, NO. 2 
