T..GOODBY 39 



The day following this experiment two more were set up. In one I 

 placed a drop containing many active larvae on the surface of a piece 

 of skin taken from a young rat's abdomen as on the previous day and 

 under the same experimental conditions. The jar with its floating raft 

 of cork carrying the skin stretched over the hole with the saline in contact 

 with the lower surface of the skin was placed in the incubator at 37° C. 

 but the stopper was out of the jar. Examination showed the larvae in 

 active movement, wriggling downward, their anterior ends pressing 

 against the surface of the skin. Several examinations were made during 

 the course of practically two hours and each time the larvae could be 

 seen actively in motion. The last time the preparation was examined, 

 i.e. after 1 hour and 55 minutes, it was found that the drop had com- 

 pletely disappeared and, owing to the conditions of illumination, it was 

 difficult to make out the details of the surface of the skin. I therefore 

 placed a drop of clean distilled water on the surface of the skin. There 

 could now be seen a large number of empty sheaths and one or two 

 freely moving larvae. Practically all the larvae, however, had left their 

 sheaths and penetrated the skin, where they could be seen moving 

 slightly when the preparation was suitably illuminated. The drop of 

 distilled water was removed by means of a pipette, and when examined 

 later on was found to be rich in empty sheaths. The skin containing the 

 larvae within it was fixed in hot 70 per cent, alcohol and the next day 

 a portion of it was divided into two layers, for the epidermis and the 

 immediately subjacent layers separated very easily from the dermis, 

 and the upper epidermal layer was cleared in lactophenol and mounted 

 whole. In this way a most interesting sHde was obtained showing empty 

 sheaths on the actual surface of the skin whilst just below, embedded 

 in the epidermis, could be seen numerous larvae which had penetrated 

 the skin. 



In the other experiment, which ran concurrently with that just 

 described, I used a piece of skin from the back of a young rat. The skin 

 was pinned on to a sheet of cork as before, and instead of putting the 

 drop of water containing the larvae directly on the skin, I placed on the 

 latter a small piece of clean blotting-paper. This was done to imitate 

 in a miniature way Looss's ((4), p. 519) experiments, in which he appUed 

 a pad of sacking or gauze to a dog's skin and then put the active 



the larvae could not get out of their sheaths. They required the presence of a downward 

 pressure of the coverslip before they could obtain the necessary purchase to break the 

 anterior end of the sheath open and then creep out. I repeated this experiment with. 

 Necator larvae and obtained the same result. 



