1 (>4 Heligmosomum muris Yokof/awa 
7. In the lungs the larvae increase rapidly in size and moult just before 
they migrate to the intestine. Early in the development in the lungs the sexes 
can be distinguished by: (1) the migration toward the posterior end of the 
genital primordium of the female, (2) structural differences in the caudal 
region, and (3) differences in shape of the genital primordium. 
8. After reaching the intestine the larvae grow rapidly and enter into the 
third moult from 96 to 108 hours after infection. In the fourth larval stage 
between the second and third moults growth and differentiation are most 
marked. It is during this stage that the differentiation of the organs of the 
reproductive system occurs. 
9. Shortly after the completion of the third moult sexual maturity is 
reached and later the cuticula separates into two layers. 
10. During the course of development the changes in size and shape-and in 
the character of the cuticula were traced step by step and the differentiation of 
the digestive and excretory systems were followed as completely as the material 
would permit. However it was in following the details of the development of 
the reproductive organs that the investigation was most fully carried out. 
11. In the male reproductive system the testes, vas deferens, seminal 
vesicle, cement gland and ejaculatory duct arise by differentiations of the 
genital primordium and are therefore called internal sex-organs, while the 
bursa and the spicules which are not developed from the genital primordium 
are known as the external sex-organs. 
12. Toward the end of the third larval stage (first parasitic stage) the 
genital primordium of the male becomes separated into two parts by an 
extremely delicate strand of tissue. The anterior half of this genital primordium 
grows forward up to the oesophageal region and forms the testes, the narrow 
strand connecting the two parts develops into the vas deferens, and the 
posterior part grows backward to the posterior end, becomes tubular and 
forms the seminal vesicle, cement gland and ejaculatory duct. 
13. The bursa is formed from the walls of the posterior end of the male 
which become very much inflated, and the spicules develop from secretions 
of a group of spindle-shaped cells which are early differentiated in the posterior 
region. 
14. In the development of the female reproductive system the ovary, 
oviduct, seminal receptacle, uterus and the anterior part of the ovijector 
arise from the differentiation of the genital primordium and are therefore called 
internal sex-organs, while the vulva, vagina and posterior part of the ovijector 
arise from invagination and differentiation of subcuticular cells of the posterior 
end and are therefore called external reproductive organs. 
15. After the genital primordium has migrated backward to a position 
on the ventral side just in front of the anus, it elongates very greatly and grows 
forward. The anterior part remains as a solid mass of cells and differentiates 
into the ovary. The rest of the primordium becomes tubular and differentiates 
into the oviduct, seminal receptacle, uterus and ovijector. 
