33 
sides are quite straight and parallel and separated by a slight interval 
(pi. XIII, fig. 2 ), although apparently attached in some manner, since 
their relative position is unchanged by movement of the fluid con- 
taining them. Next the two become more rounded and increase 
slightly in size. The nuclei are large and vesicular and the dividing 
line still persists (pi. xiii, fig. 3). Later one vermicule grows longer 
and larger and begins to partly encircle the smaller, which becomes 
more rounded (figs. 4-7). They ma}^ now be properl}^ regarded as 
gametes. By analogy the smaller and more finely granular is the 
microgamete, the larger and more coarsely granular the macroganiete. 
Eventually the line of demarcation between them disappears. The 
zygote thus formed by fusion of the male and female elements, is 
slightly elongated, tapered at one extremity, with granular proto- 
plasm and two large vesicular nuclei. The latter approach one 
another, touch, and become blended. After the union of two sexual 
elements in the manner described the zygote becomes more elongated 
(figs. 11-13) and exhibits very slow movements of flexion and exten- 
sion and something resembling ameboid motion. The term ookinet 
is applied to such a zygote. 
The cokinet continues to increase in size and becomes more and 
more markedly granular, especially around the nucleus, which con- 
tains a small karycsome. AYlien first formed the ookinet measures 
about 40 by 25 micra. It does not appear to detach any of its sub- 
stance nor does it contain pigment. From tliis point development 
is rapid. The ookinet becomes an elongated or oval body with 
granular protoplasm and a vesicular nucleus containing a moderate 
sized karyosome (fig. 13). The nuclear network is quite distinct in 
both fresh and stained preparations. In mites forty-eight hours 
after infection the ookinet has reached full size, which averages about 
25 by 50 micra. 
In sections of mites made at this period the ookinets are fairly 
numerous in the lumen of the gut and appear in many instances more 
or less curled up (pi. xii). A similar form is noted in fresh prepara- 
tions, as well as short spindle and ovoid shapes. They often appear 
somewhat flattened and occasionally very long and slender. In mites, 
three and four days after feeding on infected rats, many of the 
ookinets have disappeared, having doubtless been passed out of the 
intestinal tract with the feces. A few, however, are adherent to the 
epithelium of the gut or embedded in the gut wall between the 
epithelial cells. The protoplasm is distinctly granular and stains 
much more deeply with iron hematoxylin than do the cells lining the 
digestive tract. The karyosome is prominent and deeply stained. 
Some parasites are spherical and lie directly upon the basement 
membrane, surrounded by epithelial cells. Others are seen outside 
50519— Bull. 46—08 3 
