154 
Heligmosomum muris Yokogciwci 
In the beginning of the second parasitic stage, the internal reproductive 
system of the male shows little, if any, differentiation, and appears as a tube 
of clear cells, separated into two parts by a narrow strand of tissue. In the 
course of the second parasitic stage (Fig. 18, agp and pgp, Plate X) the cells of 
its posterior part become larger and clearer than those of the anterior one, and 
become arranged in two rows, while the cells of the anterior part remain 
small and are arranged irregularly. Then the narrow part becomes gradually 
shorter and broader. Finally all parts of the internal sex-organs become clearly 
differentiated. 
Development of the testes. It is interesting to note that the testis in H. muris 
develops from only the anterior half of the male genital primordium and the 
other internal organs from its posterior half. As mentioned above, the anterior 
part of the genital primordium is separated from the posterior part by an 
rv 
Text-fig. 6. Variations in the anterior end of the testis of the adult male. 
extremely delicate strand of tissue, and at the beginning of the first parasitic 
stage the cells of this part are not yet differentiated (Fig. 17, Plate IX). During 
the second parasitic stage the cells of the anterior part of the genital primordium 
gradually become smaller and more numerous, while those of its posterior part 
increase in size. Toward the end of the second parasitic stage and especially 
during the last moult the anterior part which forms the testis develops very 
rapidly, crossing the chyle intestine at the middle of the body and extending 
anteriorly along the dorsal side of the intestine. After the completion of the third 
ecdysis, the testis is fully developed and spermatozoa can be distinguished at 
its proximal end. Its anterior end reaches nearly to the base of the oesophagus, 
forming a loop of various shapes (Text-fig. 6). The free anterior end of the testis 
is usually more blunt and broader than that of the ovary. 
Development of the vas deferens. The vas deferens develops from the narrow 
region which is situated between the anterior and the posterior parts of the 
