162 
Heligmosomum muris Yokocjawa 
(Fig. 24, Plate XI). At the completion of the second moult they invaginate 
and push their way into the genital primordium, dividing it into two parts 
(Fig. 25, eso). The posterior portion of the genital primordium becomes very dis¬ 
tinctly separated from its main part by this invagination. It gradually decreases 
in size with the development of the external sex-organs and finally disappears 
(Figs. 25-27, jpdr). The invaginated cells increase in size and number and fuse 
with the cells of the distal end of the ovijector (Fig. 26). At this time, the cells 
which form the external sex-organs become arranged in a definite manner and 
the vagina and vulva become differentiated (Fig. 27). 
The vulva. Seven to eight cells situated at the entrance of the invagination 
are high columnar or conical in shape and form a funnel-shaped opening 
(Figs. 27 and 28, v). With the development of the body, these cells gradually 
become larger and show hook-like projections from their protoplasm (Figs. 28 
and 29, v). The cells with projections are usually three to four in number on 
each side of the opening. During the last moult these cells become cuboidal 
in shape and lose their projections (Fig. 30). At this time there is no opening 
through the cuticula into the vulva. Just before the last moult is completed 
the vulva opens through the newly formed cuticula. The new formed skin 
becomes loose, showing small folds at the entrance to the vulva. After com¬ 
pletion of the third ecdysis, the vulva opens directly toward the outside 
(Fig. 31, v, Plate XI). The new formed skin becomes much looser and extends 
posteriorly, forming a peculiar sac at the posterior end of the female. 
The vagina. The vagina develops from the invaginated cells mentioned 
above. When the vagina first becomes differentiated from the vulva it consists 
of two to three pairs of cuboid cells which connect with the posterior end of 
the ovijector (Fig. 27, vg). In the course of the second parasitic stage the 
vagina develops gradually, forming a tube, and its anterior end extends a 
little upward along the distal portion of the ovijector, forming the second part 
of the ovijector (Fig. 28, vg and foj). Toward the end of the second parasitic 
stage, the vagina develops rapidly. The cells in its wall increase in size and 
are very irregular in shape (Fig. 29, vg). They then become flattened and 
difficult to see, undergoing a granular degeneration. Finally the wall of the 
vagina appears to be formed of cuticular substance and is transversely 
striated (Figs. 30 and 31, vg). These striations appear first in the second part 
of the ovijector and then develop along the whole wall of the vagina. Until 
the completion of the third ecdysis, the vagina and the vulva are clearly 
distinguished, while in the adult stage they are not separated. Accordingly, 
the vagina of the adult worms is much longer than that of those in the fourth 
parasitic stage. 
V. ADULT. 
W hile studying the life-history of H. muris, I found some points of adult 
structure which were overlooked in my previous paper (1920). I will here 
make them clear. 
1. As noted above, the cuticula of the adult worm consists of two separate 
