308 ENTOZOA. 



Development and migration. — The precise manner in whicli tte 

 young gain access to tlie human body is very imperfectly under- 

 stood. It is well known that quantities of the immature ova are 

 expelled their "host" per anum, and I have myself obtained the 

 characteristic eggs from matters ejected by the mouth. Richter's 

 and Davaine's experiments go to prove that after the ova have 

 escaped passively, they complete their development in open waters ; 

 and it would also appear that an interval of six months must elapse 

 (after their expulsion) before the yelk-segmentation and consequent 

 embryonic formation can take place. In Richter's experiment 

 none of the embryos had emerged after the eggs had been in the 

 water for a period of eleven months ; and in the case of A. margi- 

 nata from the dog, Yerloren's previous investigations have shown 

 that the young embryos can retain their vitality for more than a 

 year after their worm-like condition has been attained. According 

 to Davaine (" Comptes Rendus," 1858, p. 1217), the fully-deve- 

 loped embryo is cyUndrical, its length being j^^ih of an inch ; the 

 mouth is not furnished with the three characteristic papillae of the 

 genus, and the tail terminates suddenly in a point. 



The experiments of Davaine also show that their development 

 in ovo was not facilitated by increase of temperature, neither were 

 the mature eggs affected by several days' immersion in the gastric 

 juice of rabbits and dogs. Further researches, therefore, are required 

 to decide whether the young Ascarides eventually gain access to 

 our bodies after the embryos have escaped the eggs and have 

 undergone a series of active wanderings elsewhere, or whether, as 

 seems more probable, they are not directly transferred from river 

 and pond water to the human stomach. 



With the view of ascertaining the exact mode in which we 

 obtain the larvae of these parasites, I have myself conducted a series 

 of experiments, though, at present, the results are rather imperfect. 

 In the case of Ascaris lumbricoides, I have only succeeded in pro- 

 curing the earlier stages of embryonic formation. Prom Ascaris 

 marginata I have reared great numbers of complete embryos, none 



