244 PARASITES OF MAN 



Notwithstanding the advantage which the size of this entozoon 

 affords us in the matter of observation and experiment, we are yet 

 ignorant as to the precise mode in which the young gain access to 

 the human body. From what has been said respecting the quick 

 growth of Ascaris mystax in the dog, and from what has been 

 observed respecting the rapid growth of the so-called A. sail In 

 in the hog, we know that the worm requires but a short time to 

 pass from the larval to the sexual state. The view of Hering, 

 Mosler, Davaine, and others, who suppose that these worms are 

 reared in a direct manner by swallowing the ova, is, as Leuckart 

 observed, not yet proved. We are not in full possession of 

 the facts of larval development. It is true that Professor 

 Heller's interesting " find " has shown that when these worms 

 first gain access to the human body their size is quite insigni- 

 ficant. At the post mortem of an imbecile, Heller discovered 

 eighteen young worms, varying in size from about J" to 5" in 

 length (2*75 to 13 mm.). The sexes were indistinguishable. 

 As a set-off against this, Leuckart's repeated attempts to rear 

 Ascaris lumbricoides and A. mystax by means of direct feeding- 

 experiments with the eggs all failed. Thus, we are yet left in 

 doubt as to the destiny of the larvae during the period which 

 elapses between the time of their escape from the egg and the 

 time of their entry into the human body. So important is the 

 question as to the mode of origination, growth, and subsequent 

 development of the larvae, that it may be well to trace, however 

 briefly, what steps have been taken to clear up the matter. 

 Leuckart obtained his negative results by the administration of 

 ripe ova to dogs, rabbits, swine, and mice. The eggs of 

 Ascaris lumbricoides have been kept alive by Dr Davaine for a 

 period of more than five years. I have myself watched the 

 development of their contents in fresh water through all the 

 stages of yolk- segmentation up to the stage of an imperfectly- 

 organised, coiled, intra-chorional embryo, and have kept them 

 in the latter condition for a period of three months. Accord- 

 ing to Davaine (' Comptes Rendus/ 1858, p. 1217), the fully- 

 developed embryo is cylindrical, its length being ^th of an 

 inch. The mouth is not furnished with the three characteristic 

 papillae of the genus, and the tail terminates suddenly in a 

 point. Davaine administered some of his five-year-old embryos 

 to rats, and had the satisfaction of finding a few of these eggs 

 in the rodent's faeces, with their embryos still living, but 

 striving to emerge. He also gave eggs to a cow, and intro- 



