192 F. KICEBLE AND F. W. GAMBLE. 



WLeu, liowover^ iufection takes place from ordinary sea- 

 water, wlien, for example, young Convolutas raised under 

 sterile conditions are placed in a few cubic centimetres of 

 sea-water brought fresh from the shore, it is not the green 

 Ihigellated cell which constitutes the first stage of infection. 

 The first sign of the presence of the infecting alga under 

 these normal conditions is afforded by a larger or smaller 

 colourless body lying in the central vacuole of the animal. 

 The large body consists of a pair of closely apposed resting- 

 cells, such as have been described already (Section IV). The 

 small body is a single resting-cell of the infecting alga. Soon 

 after ingestion the mucilaginous wall which surrounds the 

 resting-cell swells considerably and the contents divide, in 

 the case of the smaller cell into four, in that of the larger 

 cell into as many as eight, colourless daughter-cells. These 

 colourless cells are 15-16 /i in length — that is, of about the 

 same dimensions as those of the macrocytes. They escape 

 or are discharged from the central vacuole, and take up 

 fairly definite stations in the body of Convoluta, two right 

 and left of and a little behind the otocyst, and two on either 

 side about the middle of the body. These cells, which are to 

 form the starting-points from which the green tissue of the 

 adult animal will be developed, lie each, like the mother-cell 

 which gave rise to them, in a clear vacuole. The colourless 

 cell at this stage has very granular contents, and a pyrenoid 

 which is large and somewhat oily-looking. Even in this 

 stage the subsequent differentiation of the protoplast into 

 chloroplast and colourless protoplasm may be indicated by a 

 })liig or core of clearer protoplasm lying in the hollow of a 

 more granular chloroplast. The cell now develops rapidly, 

 an eye-spot makes its appearance, and a yellow tinge becomes 

 visible in the leucoplast; at first extremely faint, the yellow 

 becomes more marked, and is succeeded by a green colour 

 which pervades the whole chloroplast. The cell is bounded 

 by no well-marked wall, the limiting layer does not give a 

 cellulose reaction, and is so delicate that the shape of the cell 

 chantjes wiih the movements of the animal. 



