Insects and Parasitic Diseases. 37 



Development. 



The eggs and enibrvos of Haluronema microstoma as found in the 

 :adult Avorm are illustrated in Fig. 18. The egg (Fig. 18a) 

 jneasures from 0.04 mm. to 0.05 mm. in length by about 0.01 nun. in 

 'diameter, and is similar to that of H. muscae. Fig. 18, b and c, 

 show progressive stages in which the developing embryo is clearly 

 seen within the egg-shell. The final stage (Fig. 18 d), within the 

 uterus of the female is reached when the embryo attains a length 

 of from 0.085 to 0.105 mm., and a diameter of from .0055 mm. to 

 ■0.0075 mm. At 0.035 mm. to 0.045 mm. from the anterior end 

 there is a more or less conspicuous nucleus and frequently two or 

 three smaller nuclei may be made out nearer the posterior end. 

 'The form o^f the anterior end can be made out only with difficulty. 

 There appears to be a rudimentary pharynx and a small horn-like 

 process in front of the head, as in H. muscae, but the clear space 

 •sometimes seen surrounding what appears to be the pharynx in that 

 •.species has not been observed in H. microstoma. 



Development of the embryo in Jaeces. 



There can be little doubt but that embryos leave the horse 

 in the faeces, but it is not known whether they undergo any further 

 •development during their passage through the intestines. Definite 

 development does take place, however, in the faeces after egestion, 

 but whether this development is necessary before the embryo is 

 capable of entering the fly larvae is not yet known, owing to the 

 great difficulty of being sure of the species of the larvae found in 

 the fresh faeces, material not having been available from a horse 

 which could afterwards be known to have contained H. microstoma 

 only. 



Fig. 19 shows an embryo which was taken from the uterus of 

 an adult worm on December 21st, and incubated in sterilized 

 faeces until December 26th (see Experiment No. 12, p 31). T*' - 

 parasite, Avhich was very active, was at the end of this five days' in- 

 cubation 0.115 mm. long by 0.007 mm. wide. About 0.026 mm. from 

 the anterior end there was a group of small nuclei which were not 

 seen in other parts of the body, and at 0.053 mm. from the anterior 

 end there was a m-uch larger and more conspicuous nucleus. The 

 sheath which enveloped it Avas vei-y thin and elastic. 



In another similar culture (Expei'iment No. 8, p. 30), in wliich 

 the embryos had been incubated for aj period of three days (Novem- 



