REPORT ON THE DISEASES OP SILKWORMS IN INDIA 39 



dehisce — when the polar filament is ready to be extruded and the 

 planont is ready to emerge and infect the gut-cells. Before this 

 point is reached the digestive juice of the caterpillar has no effect 

 on it and the spore in this unripe state would pass through the gut 

 unaltered. An attempt was made by Hutchinson (1920) to count the 

 unmber of spores that a caterpillar had to ingest before it becomes 

 infected with pebrine, but his method was rather rough and the 

 number obtained over 5,000 is much too high in all probability, 

 unless he was dealing with a batch of spores that had a very great 

 many unripe ones present. I attempted the same thing by feeding 

 definite quantities of various emulsions and actually counting in a 

 haemocytometer the number of spores that would be thus ingested, 

 and I found that 400 spores produced a heavy infection. Allowing 

 for unripe spores and planonts which failed to reach the gut- wall 

 it obviously does not need a large number of spores to secure infec- 

 tion—another demonstration of the fact that a caterpillar seems 

 fairly easy to infect. 



Granted then that there is no great difficulty in infecting a 

 caterpillar — that there is no great resistance to the initial attack of 

 the parasite — how does the caterpillar act towards the parasites 

 once they are established ? We have seen, that the parasites 

 multiply with great rapidity — the gut can be infected from one 

 end to the other in four or five days (Plate VII, fig. 2) — and that the 

 worm is tolerant of an enormous infection, Does this mean that 

 here too the caterpillar has no very adequate defence against Nosema 

 attack ? The mere fact that the parasite can multiply so rapidly, 

 and be present in such enormous numbers would seem to show 

 that the more passive defences are unable to hold this particular 

 parasite in check to any extent. There are, however, two indications 

 that the caterpillar does react to a heavy infection to some extent. 

 We have already noted the budding off of part of the gut-wall 

 cells as digestive juice buds. This, while leading to auto-infection, 

 also helps to throw off a certain number of parasites, for the buds 

 will contain many meronts and many unripe spores which are not 

 capable of producing auto-infection, and which will thus be passed 

 to the outside. In certain cases of very heavy infection I have 

 observed what appears to be an intensification of this process. 

 The epithelial cells in large areas of the gut-wall had budded off 

 portions to such an extent -that they were reduced to the merest 

 fragments fringing the basal membrane. The gut thus presented a 

 most curious appearance, looking as if it had attempted to throw 

 off the parasites by budding off the epithelial cells that contained 

 them. A much commoner response to Nosema attack is seen in the 

 blackish spots which ha ve_ given the name to the disease pebrine 

 (Plate I, figs, 2, 4, 5). Their origin seems to be as follows : — -Foci 



