OF HETEROGENESIS 205 



two kinds of units very commonly co-exist, 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 ; and then, when Diatoms are 

 present, they also are of various sizes. 



5. Old, partially empty spaces are often to be seen containing 

 the Chlorochytrium fission products, small or large ; others may be 

 found containing Diatoms, small or large ; and others again partially 

 empty, but containing a mixture of the Algoid fission products with 

 Diatoms of a corresponding size. 



6. Other spaces, still densely filled, will show, in association with 

 the Algoid cells, Diatoms, either packed in their midst or occupying 

 the boundaries of spaces, and often differing greatly in size in the 

 two situations. They are likewise to be found occasionally in the 

 narrow spaces between contiguous spherical cells, where Algoid 

 spores from the parent brood not unfrequently penetrate. 



7. In the spaces where the Algoid cells and the Diatoms are 

 mixed, some of the cells may be seen to have assumed the brownish- 

 yellow colour of the Diatoms ; and some of such cells may also be 

 seen more or less elongated, and apparently developing into Diatoms. 



8. The majority of the Diatoms have an immature appearance. 

 The siliceous envelope, in the great majority of them, seems to be 

 either absent or very imperfectly developed ; and unmistakable 

 evidence that multipUcation of these immature Diatoms has taken 

 place is frequently to be seen.' 



There is no probability, and no one, I think, is likely to maintain, 

 that Diatoms are normal phases in the hfe-history of this parasitic 

 Alga ; and as a careful consideration of the evidence, as a whole, 



• Some of the differences in size, apart from those due to differences in the 

 size of the Algoid fission products from which the Diatoms originated, may be 

 owing to actual increase of bulk in these immature organisms. Although this 

 supposition is at variance with commonly received views, it is in accord with the 

 observations of Geo. Murray, who saj's [loc. cit., p. 216) that young Diatoms 

 formed within a parent by a process of rejuvenescence, when liberated by " the 

 separation of the parent valves at the girdle, may grow, divide and multiply 

 before fully attaining the characteristic external sculpturing and adornment of 

 the parent." Young Diatoms originating in fresh water may find silica in all 

 pond water. The ammonia contained in rain water, like other alkalies, easily 

 dissolves silica or aluminium silicate when in a finely pulverised state, and one 

 or other of these compounds is to be found in all soils. (See Prof. A. M. Edwards, 

 " On the Solubility of Silica," " The Chemical News," January, 1896, p. 13.) 



