History of the Nematode Worms. 455 
worms are seen with an obtuse tail. In the periphery of the 
buccal orifice there are three small lip-like processes, enclosing 
a small, almost spherical buccal cavity, with a strong chitinous 
wall. In the following days the size of the body increases con- 
siderably. At the end of the first week, the worms not unfre- 
quently measure as much as | millim.; in the second week they 
increase to 3°5 millims. All the individuals do not, however, 
grow equally fast; among the larger ones we occasionally find 
small specimens measuring scarcely two-thirds of the former, as, 
indeed, is not unfrequently observed in helminthological experi- 
ments. The increase in thickness at first pretty nearly keeps 
pace with that of the length. At 0°85 millim. it is about 0-04, 
and at 2 millim. about 0:09. But when the worm has attained 
the latter size, and the intestine, previously slightly coloured by 
the browning of the epithelial cells, begins to be filled with 
blood, the transverse diameter of the young worms increases 
considerably, so that the original slender form gradually gives 
place to a shorter and stouter one. Worms of 3°5 millims. are 
fully 0:16 millim. in thickness, and those of 5 millims. in length 
(such as are met with towards the end of the third week) are as 
much as 0:23 millim. thick. The extreme caudal point alone 
takes no part in this thickening. It remains thin and slender, 
as in the worms of the first week, and then, as might be ex- 
pected, is very sharply marked off from the rest of the body, 
almost in the form of a spine. 
This increase of size of the young A. nigrovenosa does not, 
however, take place without repeated sheddings of the cuticle. 
This is most distinct in the later stages, where the swelling body 
is usually surrounded by the remains of the cast-off and wrinkled 
skin, as if by a scaly covering. Of course, also, the cuticle con- 
stantly becomes thicker with the increase of the size of the body, 
and the musculature makes its appearance more and more dis- 
tinctly. 
The differentiation of the sexual organs commences even be- 
fore the length of the body has reached 1 millim. About this 
time the vagina may first be perceived, a little behind the middle 
of the body, with two horns running from it forward and back- 
wards, which are at first short and thin, and terminate cecally 
at a distance of about 0°07 millim. from the genital orifice. In 
individuals of 2 millims. in length each of the two genital tubes 
measures about 0:3 millim. ‘Their course is very irregular, so 
that the end is scarcely 0-2 millim. from the orifice. The in- 
ferior section of the tube is the thickest (0-025 millim.), and is 
distinguished by an internal epithelial coat. A little further up 
we see a portion with delicate annular fibres ; and this is followed 
by the longest and thinnest section, the true ovary, in the cecal 
