ASCAEIS MISTAX. 329 



Ascaris megalocephala, but I could not speak confidently as to the 

 precise manner in whicli the spermatozoa were brought in contact 

 with the granular yelk. The union of the sexual elements is 

 quickly followed by a condensation of the yelk granules and the 

 disappearance of the hitherto centrally placed germinal vesicle. 

 The ovum next assumes a distinctly oval form, the true yelk 

 membrane and the external chorion, or shell membrane, now 

 becoming more and more distinct, until the latter acquires a 

 regularly tuberculated surface, and the egg finally assumes its 

 characteristic figure. Co-ordinating with these changes the 

 granular yelk is seen to develop itself into a large embryonic cell, 

 which, after a while, divides and subdivides by the well-known 

 process of yelk- segmentation, until, at length, we have the worm- 

 like embryo developed and coiled upon itself in various ways. In 

 Ascaris mystax the young does not make its escape from the shell 

 until after the escape of the egg from the body of the parent ; but, 

 I need hardly add, there are many viviparous nematodes in the 

 oviducts of which one may see the young moving about freely, 

 during Hfe. As examples, I may mention Dracunculus medinensis, 

 Strongylus paradoxus, and Oucullanus elegans. 



22. Trichocephalus dispab. 



T. dispar, Rudolphi ; Bradley ; Bremser ; etc. 



T. hominis, Goeze ; Schrank ; Gmelin ; etc. 



T. simice patas, Treutler. 



T. lemuris, Rudolphi. 



T. palceformis, Rudolphi ; Dujardin. 



Trichuris, Buttner and Roederer ; Wagler ; Bloch. 



Ascaris tricUura, Linneus ; Miiller ; Werner. 



General and Specific Characters.— A. small nematode helminth, the male measuring 

 one inch and a half in length, and the female fuUy two inches ; characterized especially 

 by an extremely long filiform neck, occupying about two-thirds of the entire length of 

 the body ; surface of the skin smooth to the naked eye, but, when highly magnified, 

 seen to be furnished on one side with a longitudinal band of minute wart-like papillffl, 



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