24 Gerald F. Hill: 



The most noticeable feature of its development compared with 

 earlier stages was its greatly increased length in proportion to 

 increase in width. 



The length of this larva was 0.65 mm. and the width at the pos- 

 terior end of the oesophagus 0.05 mm. The posterior end of the 

 oesophagus was 0.23 mm. from the anterior end of the body, and 

 about the junction of the first and second third of the oesophagus 

 there was a fairly conspicuous group of rather large nuclei indicat- 

 ing the position of the nerve ring. 



Excepting in a general increase in size no marked structural 

 development was observed in the parasites shown in Figs. 10 and 

 11, which Avere found on December 7th in fly pupa from the same 

 culture as that which produced the parasites shown in Figs. 5, 6, 

 7 and 9. These larvae were, therefore, not more than eleven days 

 old. 



The vsmaller parasite (Fig. 10) was 0.8 mm. in length, the larger 

 (Fig. 11) about 0.83 mm. In each the posterior end of the oeso- 

 phagua was about 0.28 mm. from the anterior end of the body, the 

 maximum width of which was 0.05 mm. The distance of the nerve 

 ring from the anterior end was about 0.1 mm. and that of the anus 

 from the tip of the tail about 0.060 mm. 



Further than a general increase in size, no marked development 

 was seen in the larvae shown in Figs. 12, 13, and 13A, as compared 

 with those of earlier stages. 



Fig. 12 represents a larva found in a thin-Avalled spherical cyst 

 in the abdomen of a fly pupa in which the adult fly was almost 

 ready to emerge. The embryo from which it was derived was one 

 of many obtained from ten gravid females on December 21st, and 

 incubated in sterilized faeces at I'oom temperature for a period of 

 six days before being given an opportunity of infecting fly larvae. 

 The fly larvae used in this experiment were about nine days old, 

 and nearly mature when they were liberated on December 27th in 

 the embryo-infected faeces. The resulting pupae were examined 

 on January 3rd; the parasite was therefore about thirteen days old, 

 and had spent not more than seven days of its life within the body 

 of the fly larva. 



A larva in the same state of development as that referred to 

 in the preceding paragraph, is sliown in larger scale in Figs. 13 

 and 13A. It was found on November 30th free in a fly pupa from 

 the culture referred to in discussing the embryo shown in Fig. 4. 

 The age of this parasite and the fly pupa was, therefore, nine days, 

 the first 42 hours of which period liad been passed by the nematode 

 embryo in sterilized faeces apart from the fly larva. 



