Fission-products of a Parasitic Alga. 181 
diatoms are never to be seen in the spaces in which the 
Chlorochytrium is in one of its early stages of development ; 
they are to be found only in association with its later stages, 
where some of the final segmentations have been taking 
place, and often where the patches are so old that the walls 
of the spaces containing them are stained of a rust-colour. 
7. None of the diatoms found either within the spaces or 
within their ramifications between surrounding cells have 
ever been seen to move. 
8. Moreover, where the diatoms exist they are often inti- 
mately intermixed with the algoid cells; they are also to be 
seen in the peripheral regions of spaces, even when these are 
still full, and small specimens are likewise to be found between 
the spherical subepidermal cells contiguous to the invaded 
space. Such facts are incompatible with an entry of diatoms 
from without, especially if we bear in mind what has been 
said under the last two heads. 
9. Again, where the diatoms exist they not only vary much 
in size and shape in different spaces, but even within different 
regions of the same space. 
Taken as a whole these various facts seem to me abso- 
lutely to negative the Infection Hypothesis as a means of 
accounting for the association of the diatoms with the fission- 
products of Chlorochytrium in the subepidermal spaces. 
(b) Transformation Hypothesis.—The facts which are so 
incompatible with the foregoing hypothesis will be found 
either to offer no difficulties to, or to be capable of receiving a 
ready explanation in accordance with, the transformation 
hypothesis. This hypothesis is also strengthened by other 
facts not previously reterred to. 
1. The absence of the diatoms from the 90 per cent. of 
the substomatal spaces which are not infected by the alge is 
explained. 
2. The absence of movements on the part of the diatoms in 
question affords no difficulty. 
3. The absence of the diatoms from the Chlorochytrium 
spaces during the early stages of the development of the alga 
affords no difficulty and is explained. 
4, The variation in the size of the diatoms is explained, in 
the main, by the varying size of the fission-products of the 
aloa. The two kinds of units very commonly coexist, and 
where the algoid cells are small the diatoms are small, where 
they are of medium or larger size the diatoms are similarly of 
medium or larger size. Such variations in the size of the 
algoid cells are very common within the same infected space, 
