152 
Heligmosomum muris Yokogawa 
2. The subcuticular layer of the ventral wall just in front of the anus of 
the male does not at this time show any cell-divisions, while in this region of 
the female already three or more cells can be distinguished. 
In the youngest free-living stages the genital primordium apparently con¬ 
sists of two cells. It is a small ovoid body, measuring about 0-006 mm. by 
0-004 mm. It develops gradually with the development of the body. In 
the mature free-living larvae the genital primordium consists of six to eight 
cells and has a size of 0-012 mm. to 0-017 mm. by 0-006 mm. to 0-008 mm. In 
this stage its shape varies and it almost always has at its anterior end a 
small more or less dark body as is shown in the figure (Text-fig. 2). In a few 
cases this unknown body was found at the posterior end of the genital pri¬ 
mordium. After infection, the genital primordium becomes gradually trans¬ 
formed in size and in general appearance according to the stage of development 
and early shows sexual differences. In the female it appears spindle-shaped 
in the first parasitic stage, while in the male it is rather oblong in shape 
(Text-figs. 5 and 7). This difference is clearly defined 24 hours after infec¬ 
tion. At this time the small darker body described above is found usually 
at the anterior part of the genital primordium of the male, while it rarely 
can be seen in the female. 
(c) The Development of the Male Reproductive Organs. 
(1) The Internal Sex-Organs. 
General description. The term “internal sex-organs ' is used to refer to 
the organs which develop from the genital primordium, that is, the testis, 
vas deferens, seminal vesicle, cement gland and ejaculatory duct. As stated 
in the previous section, the genital primordium of the male is situated ventrally 
near the middle of the body. Toward the end of the first parasitic stage it 
becomes separated into two parts by an extremely delicate strand of tissue. 
The beginning of this division can be distinguished early by a slight constric¬ 
tion in the middle of the genital primordium (Text-fig. 5). The division of 
the genital primordium usually occurs in 40 to 50 hours after infection and 
sometimes later. After division the posterior part develops more rapidly than 
the anterior one growing toward the posterior end of the body. When the 
young male enters the second parasitic stage the end of this posterior division 
of the reproductive primordium has grown back to the end of the chyle 
intestine. After growing to this point the anterior end of the posterior part 
of the genital primordium grows forward. Meanwhile, the anterior part grows 
anteriorly along the ventral side of the intestine. Toward the end of the second 
parasitic stage, and especially during the last moult, the anterior part of the 
reproductive body develops very rapidly, crossing the middle part of the 
intestine and growing forward. By the completion of the last ecdysis it 
reaches almost to the anterior end of the chyle intestine. 
