38 Enslieatlied Larvae of some Parasitic Nematodes 



In due course a stool was obtained containing a large number of 

 adult N. americanus together with their eggs. Cultures of the faecal 

 matter were put up with animal charcoal in large Petri dishes the lids 

 of which were provided with a layer of blotting-paper, which was always 

 kept moist. After eight days a good supply of ensheathed larvae was 

 collected from the lids by washing the blotting-paper with water. The 

 washings were centrifugalised and the larvae concentrated into a small 

 bulk of water. 



A young rat, three days old, was chloroformed and the skin from 

 the abdomen and flanks was removed. This was stretched over a hole 

 in a piece of sheet cork and pinned in position. The cork was then 

 floated on the surface of N saline warmed to 37° C. contained in a glass 

 jar as already described. 



Examination of the droj) under the microscope showed the larvae 

 in very active movement with their anterior ends pressing downwards 

 on to the skin as though trying to get into it. The jar was closed by its 

 stopper and placed in the incubator at 37° C. At frequent intervals it 

 was taken out and after removal of the stopper the drop was examined 

 under the microscope. Always the larvae were seen to be very actively 

 wrigghng in a downward direction, but at the end of two hours there 

 was no sign of any of them having escaped from their sheaths and pene- 

 trated the epidermis. I did not understand the reason for this as I was 

 under the impression that I had brought about the requisite conditions 

 for skin penetration. However, I put the jar with the cork and its 

 attached skin back into the incubator, but with the stopper left out. 

 On examining the preparation under the microscope the following 

 morning I found that the drop of water had evaporated and that on 

 the surface of the skin, within which many larvae could be seen, there 

 were several empty sheaths. From this I inferred that evaporation of 

 the water containing the larvae was necessary before they could leave 

 their sheaths and penetrate the skin. At all events it seemed probable 

 that there was some mechanical necessity for a shallow drop or even a 

 film of water rather than a globule for the larvae to act in. In a deep 

 globular drop, although they could be seen actively wriggling downwards 

 on to the skin, they seemed to lack the necessary purchase of pressure 

 upwards against a resistant surface to enable them to leave their sheaths 

 and penetrate the skin^. 



^ Looss's description ((4), p. 431) of liis lepetition of Herman's experiment on tlie 

 effect of methyl-green stain on ensheathed Ancylostoma larvae bears on this point. He 

 found that if the droji containing the larvae and the stain was not covered with a coverslip 



