386 Prof. T. H. Huxley. Pathology of the [Mar. 2, 



free when they emerge from the zoosporangia ; and, from their extreme 

 minuteness, they must be readily carried away and diffused through 

 the surrounding water. Hence, a salmon entering a stream inhabited 

 by the Sa/prolegnia will be exposed to the chance of coming into con- 

 tact with Sa/prolegnia spores ; and the probability of infection, other 

 things being alike, will be in proportion to the quantity of the 

 growing Sa/prolegnia, and the vigour with which the process of spore- 

 formation is carried on. At a very moderate estimate, a single fly 

 may bear 1,000 fruiting hyphge ; and if each sporangium contains 

 twenty zoospores, and runs through the whole coarse of its develop- 

 ment in twelve hours, the result will be the production of 40,000 

 zoospores in a day, which is more than enough to furnish one zoo- 

 spore to the cubic inch of twenty cubic feet of w T ater. Even if we 

 halve this rate of production, it is easy to see that the Sa/prolegnia 

 on a single fly might furnish spores enough to render such a 

 small shallow stream as salmon often ascend for spawning purposes, 

 dangerous for several days. But a large fully diseased salmon may 

 have as much as two square feet of its skin thickly covered with 

 Sa/prolegnia. If we allow only 1,000 fruiting hyphse for every 

 square inch, we shall have 288,000 for the whole surface, which, 

 at the same rate as before, gives over 10,000,000 spores for a day's 

 production, or enough to provide a spore to every cubic foot of 

 a mass of water 100 feet wide and 5 feet deep and four miles long. 

 Forty such diseased salmon might furnish one spore to the gallon for 

 all the w T ater of the Thames (380,000,000 gallons per diem) which 

 flows over Teddington Weir. But two thousand diseased salmon 

 have been taken out of a single comparatively insignificant river in 

 the course of a season. 



It will be understood that the above numerical estimate of the pro- 

 ductivity of Saprolegnia, has been adopted merely for the sake of 

 illustration ; that I do not intend to ^suggest that the zoospores are 

 evenly distributed through the water into which they are discharged 

 by the zoosporangia ; and that allowance must be made for the very 

 short life of those zoospores which do not speedily reach an appro- 

 priate nidus. Nevertheless, the conclusion remains arithmetically 

 certain that every diseased salmon adds immensely to the chances of 

 infection of those which are not diseased ; and thus, the policy of 

 extirpating every diseased fish as soon as possible, has ample justifi- 

 cation. But, in practice, the attempt to stamp out the disease in this 

 fashion would be so costly that it may be a question whether it is not 

 better to put up with the loss caused by the malady. 



There are many practical difficulties in the way of directly observing 

 the manner in which the zoospores effect their entrance into the skin 

 of the fish ; but, on comparing the structure of the healthy integument 

 with that of the diseased patches, the manner of the operation can 



