23, i Schwartz: Life History of the Horse Oxyurid 43 



HATCHING 



Hatching was observed in vitro in eggs five days after ovi- 

 position and one day after the actively motile embryonated 

 stage had been reached. As has already been stated, the 

 embryos escape through the opening in the shell that is covered 

 by the operculum. The latter is probably lifted by the con- 

 tinuous and violent movements of the embryo and frequently 

 drops off entirely, leaving the opening unguarded. I occasion- 

 ally observed embryonated eggs without an operculum before 

 the larva had even begun to emerge from the shell. 



Hatching was seldom observed in eggs that were kept dry 

 on glass slides. Moistening the eggs with water or salt solu- 

 tion usually resulted in hatching, the percentage of hatching 

 eggs varying considerably in different preparations. As a rule 

 relatively few hatching eggs were observed in cover-glass prep- 

 arations, but in several instances over 50 per cent of the eggs 

 in such preparations hatched. The larva emerges from the shell 

 either head first or tail first, and some forms appear to ex- 

 perience considerable difficulty in wriggling out of the shell. 

 A hatching form in which part of the larva is outside of the 

 shell and the remaining part is still within the shell is shown 

 in Plate 1, fig. 11. As the egg opening is too small to enable 

 the worm to wriggle out easily, it is seen to be constricted a 

 little above the point of insertion in the shell opening, that por- 

 tion just having slipped through. 



I have frequently observed a similar condition in the hatch- 

 ing of certain Strongylidae, and in these forms I observed a 

 constriction at the point of insertion in the ruptured portion 

 of the shell and a streaming movement of the granular larval 

 substance from the portion of the worm that was still inclosed 

 in the shell to that outside of the shell, thus rendering the former 

 more compressible and making it possible for the larva to pull 

 itself through a narrow opening. 



That hatching of Oxyuris equi in vitro is purely accidental 

 is evident from the fact that larvae which emerge from the 

 eggshells into water burst, owing to excessive absorption of 

 the fluid, thus showing conclusively that the larvae are not des- 

 tined to a free-living existence. Larvae that emerge from their 

 eggshells into physiological salt solution do not become plas- 

 moptized, but move about rather sluggishly. No signs of molt- 

 ing in vitro have been detected in these larvae. 



