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Heligmosomum muris Yokogawci 
the post-embryonal stages of AUantnema mirabile and Neuhaus (1903) studied 
these stages in a free-living nematode, Rhabditis nigrovenosa. The most notable 
study on the post-embryonal development of a nematode was made by Looss 
(1897, 1911) on the hookworm, Ancylostoma duodenale. More recently in South 
Africa Veglia (1915) has made extensive studies on the post-embryonal develop¬ 
ment of Haemonchus contortus from the sheep, and Theiler and Robertson 
(1915) have investigated these stages in Trichostrongylus douglasi, the wire- 
worm from the ostrich. In the studies which have been made of the post- 
embryonal development of the Strongylidae the free-living stages, especially 
of the hookworms, have been very completely investigated. While in only a 
comparatively few forms have the free-living stages been studied in detail, 
there is found to be a considerable degree of uniformity in the development 
and relationship of these stages, so that we have an adequate conception of 
what the stages of development of this stage are. It is entirely otherwise with 
the parasitic development in this family. The literature gives an entirely in¬ 
adequate conception of the morphogenesis of the various organ systems, and 
on some phases of this development there is no information whatsoever. This 
lack of knowledge is probably partly due to the fact that in most nematodes 
this period is of considerable length and many of the hosts studied are large 
animals. 
Heligmosomum muris, which I described from the rat (Yokogawa, 1920), 
proved to be remarkably good material for the study of development during 
the parasitic stages. This form is small and develops in ordinary culture rats 
as well as in wild rats. But best of all infection can be easily obtained and the 
development in the rat requires only a few days. This made it possible to 
obtain an abundance of material of the stages of development and to obtain 
at any time material of any particular stage on which further observations 
were needed. 
All the observations recorded in this paper were made from living speci¬ 
mens. To quiet the larvae the slide could be heated or exposed to direct 
sunlight. To find the larvae in the lungs of the experimental rats it was neces¬ 
sary to tease the tissue into very small pieces in normal salt solution. These 
pieces were then carefully crushed in a mortar and filtered through a fine 
wire screen. The filtrate was then centrifuged and the larvae if present 
would be recovered. After the larvae had made their way to the intestine 
it was not difficult to pick them from the surface of the mucous membrane. 
In the present paper no attempt is made to study in detail the embryonic 
development which many authors have studied in nematodes. While the 
free-living larval stages were studied as fully as the time would permit no 
attempt has been made to discuss critically these stages for the Strongylidae. 
Looss (1911) gives a very complete and critical analysis of these stages in 
Ancylostoma duodenale. The main emphasis in the paper is placed on the 
study of the parasitic development, and especially on the development of the 
reproductive organs. 
