THE BIONOMICS OF CONVOLUTA ROSCOFFENSIS. 389 



from without. Indeed, the capsular fauua contains numbers 

 of colourless cells similar to tliose which constitute the first 

 stage of the assimilating tissue. 



The evidence of the first appearance of the colourless cells, 

 their position in the gut, and their structure, as detailed in the 

 next section, are also in favour of the infection hypothesis. 

 They appear most constantly in cultures made from the 

 capsules, but even in these a certain capriciousness is very 

 obvious. Larva3 from the same clutch rarely contain colour- 

 less cells to the same number, or in the same stage of develop- 

 ment. Some have several, some few, some none at all. This 

 irregularity of appearance and occurrence under apparently 

 identical conditions is a strong argument against the colour- 

 less cells being normal intrinsic developments of the animal 

 tissues, and also against a plastid theory of the origin of the 

 chloroplasts independently of the cells in which they are 

 subsequently found ; but it is readily intelligible if infection 

 occurs now rapidly after birth, now only after an interval of 

 some days, and it may be not at all. 



The position of the colourless cells in the gut enclosed by 

 wandering cells, in contrast to their definitive position in the 

 more superficial parenchyma, is also in favour of the infection 

 theory, since we have seen that wandering cells carry food 

 thither. Finally, the structure of the colourless cells, as 

 given in the next section ; the changes which they undergo 

 during their conversion into assimilating green cells ; the 

 gradual enclosure of the colourless cytoplasm by one or more 

 chloroplasts which pnss through a colourless or yellowish 

 phase, and finally adopt a green tint; the occasional presence 

 of starch in the leucoplast stage ; the presence of a coloured 

 eye-spot or stigma ; the gradual development, in most cases, 

 of the power of photosynthesis, are phenomena all of which 

 occur in plants, and can be closely paralleled in certain algse, 

 whichj after a period of saprophytic life, during which they 

 have lost their chlorophyll and stored up reserves, enter upon 

 a period of ordinary holophytic nutrition (Colin, 18G4; 

 Kruger, 1894; Karsten, 1901; Dangcard, 1902). 



