S. Yokogawa 
149 
Looss (1897 and 1911) did not add to the knowledge of this point in the 
development of nematodes even though he studied on the development of 
Ancylostoma, Strongyloides and Rhabdonema. Veglia (1915) who studied the life- 
history of the Haemonchus contortus added but little to the knowledge of the 
development of the reproductive organs. Theiler and Robertson (1915), who 
worked on the life-history of the wire-worm in Ostriches, mentioned briefly 
the development of its bursa and vulva. They described the development 
of the vulva as follows: “The central ceil is the biggest of the lot of the 
female primordium; ventrally it soon splits crossways to the longitudinal 
axis, indicating the future vulva. This cell is also raised slightly above the 
adjoining ones and reaches the subcuticula. In a somewhat more advanced 
stage this slit is surrounded by a number of cells.” Their description is in¬ 
adequate and their explanation of the development of the vulva seems un¬ 
reasonable. They described also very briefly the formation of the bursa. As 
far as it goes their descriptions agree with what has been found in H. muris. 
I studied very carefully and made clear the development of the reproductive 
apparatus,-and knew that the testes, vas deferens, seminal vesicle, cement 
gland and the ejaculatory duct develop from the male primordium, while the 
bursa, spicules and the gubernacles are produced without reference to the 
genital primordium. Accordingly I shall call the former the internal sex- 
organs, and the latter the external sex-organs. I also found that the ovary, 
oviduct, seminal receptacle, uterus and ovijector (houstrix) are produced from 
the genital primordium, while the vulva and the vagina develop from outside 
the genital primordium. The second part of the ovijector is produced from the 
distal end of the genital primordium and the anterior part of the vagina, 
that is, this part corresponds to the adjoining part of the internal and external 
sex-organs. 
(6) The Beginning of Sexual Dimorphism. 
Sexual dimorphism has not been noted for the free-living larval stages of 
the Strongylidae. Looss (1911) criticized the suggestion of several authors 
who tried to distinguish the sexes in free-living nematode larvae by the shapes 
of their tails. He concluded that sexual dimorphism cannot be detected in 
the free-living larvae of the nematodes. Looss (1911) on the Ancylostoma, 
Theiler and Robertson (1915) on the wire-worm in Ostriches, and Veglia (1915) 
on the Haemonchus contortus, do not describe the beginning of sexual di¬ 
morphism, and so far as I know there is no literature on this subject. I think 
it is very interesting and important matter to learn just when and how the 
sexual differences appear during the parasitic life. Sexual dimorphism in 
H. muris was distinguished in very young larvae during the first parasitic 
stage (Text-fig. 3 a and 6). 
Three essential points of difference between the sexes were determined at 
this early stage, i.e., first, the posterior migration of the genital primordium 
of the female; second, structural differences in caudal regions; and third, 
differences in the shape of the genital primordia. 
