210 DR. COHN, ON THE DEVELOPMENT OF 



The primordial utricle retreats from the wall and contracts, 

 expelling the water from its interior. The peculiarly-arranged 

 Chlorophyll is detached, and begins to be discoloured ; at the 

 same time, the two vacuoles, filled with granules at each end of 

 the Closterium, disappear ; ultimately, the cell appears colour- 

 less and empty, merely some remains of the chlorophyll 

 being left, in the form of an irregular contracted saccular 

 mass or masses in the middle of it. That this morbid change 

 of the cell-contents is due solely and wholly to the influence 

 of the parasite, may be shown beyond all doubt. For the 

 contraction of the primordial sac begins at the spot where tlie 

 Chi/tridium spore was attached ; and according as the parasite 

 has germinated at the middle of the fnmd, or at one or other of 

 the ends, it is there that the contents first appear to suffer. 



It is clear, therefore, tliat the Cliytridmm is nourished at 

 the expense of the Closterium. ; and it may readily be con- 

 ceived, that a parasite of this kind may produce a most de- 

 structive epidemy among the millions of inhabitants in a 

 glass of water, when it is considered that each Cliytridium 

 may contain 4,000 motile spores, each of which is capable of 

 destroying a Closterium, and after a few hours may itself 

 reproduce the same number of spores. 



As regards the mode in which the fatal influence of the 

 CJu/tridium is exerted, it would seem to consist not simply in 

 an endosmotic action through the walls of the two cells, but 

 by means of a kind of radical fibres which insinuate them- 

 selves into the Closterium, and which may probably be re- 

 garded as equivalent to the mycelium of a fungus. Dr. A. 

 Braun (1. c.) expressly desci'ibes these radical fibres, as they 

 appear to have occurred several times to Cohn's observation, 

 who has given figures of what he saw. It would thence 

 seem, that the Chytridium-vesicle is attached to its organic 

 basis, not simply by adhesion, but by a myceloid tissue, 

 which penetrates the membrane of the Closterium, and ramifies 

 in its interior. 



The systematic position of this parasitic fungus, for such it 

 must obviously be regarded, unless all the Mycophyceae are 

 merged in the Algan order, is clearly in that order, in the 

 family of the Saprolegniea^, itself a section of the Vaucherieae. 



With respect to the relations of the ' swarm-cells ' of the 

 aquatic fungi to the monades. Just as the green ' swarm- 

 cells ' of the true algcE so closely resemble certain astomatous 

 green infusoria, that it is impossible in many cases to deter- 

 mine whether a doubtful form belong to the animal or to the 

 vegetable kingdom ; so do the colourless monads in form and 

 colour precisely correspond with the colourless ' swarm-spores' 

 of the Mycophycece. It is even probable that the number of 



