SEX DETERMINATION IN THE WHITE-FLY 271 
PARTHENOGENESIS 
Up to this point all eggs behave in exactly the same manner. 
The subsequent processes in the egg-nucleus depend on whether 
the egg has been fertilized or not. If no spermatozoon has en- 
tered the egg, a well-defined nuclear wall is formed around the 
matured egg-nucleus, and the chromosomes enter a semi-resting 
stage in which they become less sharp and definite in outline. 
They, nevertheless, still can be counted. Soon after, the rounded 
form is lost and the chromosomes lengthen, assuming the shape 
typical of those found in the dividing somatic cells of all the suc- 
ceeding developmental stages. The first segmentation division 
proceeds normally, each of the first two nuclei receiving eleven 
chromosomes. 
FERTILIZATION 
If the egg has been fertilized, the egg-nucleus, as in the unfer- 
tilized egg, likewise approaches the center. The condition and 
position of the spermatozoon previous to this stage is very diff- 
cult to determine. Various sections showed what may possibly 
be the sperm head in a resting condition, but identification is 
never certain, because of possible confusion with the equally 
stained yolk material. Now, however, the sperm nucleus be- 
comes apparent. At first still retaining the lengthened and 
pointed form of the sperm head, it gradually rounds out, under- 
going the ordinary process of chromosome formation. When the 
two pronuclei meet, they are equal in size and each shows eleven 
lengthened chromosomes. The nuclear walls then break down 
and the twenty-two chromosomes are inclosed in an irregular 
vesicle. The first segmentation is regular in character, as before, 
and results in two nuclei, each with twenty-two chromosomes. 
THE CHROMOSOMES SUBSEQUENT TO MATURATION AND 
FERTILIZATION 
Cytological investigation of such forms as Ascaris and the bee 
has demonstrated the possibility of formation of multiple chro- 
mosomes, that is, products of the union of two or more chromo- 
