94
Oesophagostomum radiatum
The further growth of the egg into the multi-cellular stage is completed
while the egg is being transported with the dung to the outside. If conditions
are favourable, the morula-stage is reached in nearly 2½ hours after expulsion
(Fig. 5). The blastomeres undergo repeated division and individual cells
cannot be distinguished. The egg-shell now widens to accommodate the
growing embryo. Segmentation has been total and is unequal hereafter.
At one pole, the proliferation of cells progresses at a faster pace than at the
other, and the resultant embryo attains the typical tadpole-stage with a
broad anterior and narrow posterior region, in five to six hours after evacua-
tion with the dung (Fig. 6). The surface of the embryo is coarse and there is a
pronounced bilateral symmetry in the organogeny. By a process of growth
in length and diminution in width the embryo acquires an almost uniform
thickness. Being longer than the long axis of the egg-shell this lies bent into
two (Fig. 7). Movement to and fro inside the shell is commenced. Growth
in length progresses and the head, body and tail end of the vermiform embryo
could be identified. The larva is coiled in various patterns, the anterior end
is rounded and broad, and the tail long and attenuated. There is an incessant
activity inside the shell when seven to eight hours old and no internal struc-
ture is evident except clusters of granules. In about 10 hours, the buccal
cavity appears as two parallel lines behind the mouth, the œsophagus and
the intestine as a group of dark cells, indicating the rhabditoid nature
of the former (Fig. 8).
The hatching of such an egg may be observed, on a microscopic examina-
tion, to begin from 12 to 14 hours in eggs passed in the dung as the early
development had been undergone during their passage through the intestines
of a host, whereas eggs squeezed from a female worm require 20 hours or more.
The larva moves frantically for nearly 45 minutes inside the shell and punc-
turing one pole of the egg thrusts its oral end out. But the exit of the larva
is not immediate as could be expected. The anterior end moves without
rest and there seems to be some difficulty in getting the looped tail out of the
shell. The larva ultimately escapes from it and after a pause of five minutes
commences activity. The shell retains its oval shape, unless it is rotated this
way or that by the tail of the larva, when it yielded at the place of pressure.
In respect of this softness of the shell, the opening could only be said to be a
tear and not a break. It has been found as a result of many experiments
that eggs obtained from the mother-worm are poor for culture-work in so
much as 90 per cent of them do not develop. This is mostly due to the large
proportion of immature and unfertilized ova among them.
The larval development
There are five stages in the evolution of the hatched larva into the adult,
the first, second and part of the third being free-living, and the other part of
the third, the fourth and fifth stages parasitic in the intestinal tract of the host.
The free-living third-stage is the infective larva and the fifth the adult. This
distinction is based on definite morphological differences.
The first-stage larva
The larva is of the rhabditoid type and is more or less uniform in width
till the anus, wherefrom it tapers to form a delicate and pointed tail (Fig. 9).
The length of the larva varies from 0.4 to 0.43 mm. and the thickess from