544 MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPHYTES 



or of some modification of it. Outside this (as shown at A), when 

 the ' still ' cell is formed by a change in the condition of a cell that 

 lias been previously 'motile/ we find another envelope, which seems 

 to be of the same nature, but which is separated by the interposition 

 of aqueous fluid ; this, however, may be altogether wanting. The 

 multiplication of the ' still ' cells by subdivision takes place as in 

 Palmoglcea, the endoplasm first undergoing separation into two 

 halves (as seen at B), and each of these halves subsequently developing 

 a cellulose envelope around itself, and undergoing the same division 

 in its turn. Thus two, four, eight, or sixteen new cells are succes- 

 sively produced ; and these are sometimes set free by the complete 

 dissolution of the envelope of the original cell ; but they are more 

 commonly held together by its transformation into a gelatinous 

 investment, in which they remain imbedded. Sometimes the endo- 

 plasm subdivides at once into four segments (as at D), of which every 

 one forthwith acquires the character of an independent cell ; but 

 this, although an ordinary method of multiplication among the ' mo- 

 tile ' cells, is comparatively rare in the ' still ' condition. Sometimes, 

 again, the endoplasm of the * still ' form subdivides at once into 

 eight portions, which, being of small size, and endowed with motile 

 power, may be considered as zoospores. As far as the complete life- 

 history of Protococcus is at present known, some of these zoospores 

 retain their motile powers, and develop themselves into the ordinary 

 * motile ' cells ; others produce a firm cellulose envelope and become 

 ' still ' cells ; and others (perhaps the majority) perish without any 

 further change. 



When the ordinary division of the ; still ' cells into two segments, 

 has been repeated four times, so as to produce sixteen cells and 

 sometimes at an earlier period the new cells thus produced assume 

 the * motile ' condition, being liberated before the development of the 

 cellulose envelope, and becoming furnished with two long vibratile 

 flagella which seem to be extensions of the colourless protoplasm 

 layer that accumulates at their base so as to form a sort of trans- 

 parent beak (H). In this condition it seems obvious that the colour- 

 less protoplasm is more developed relatively to the colouring matter 

 than it is in the * still ' cells ; and it usually contains ' vacuoles ' 

 occupied only by clear aqueous fluid, which are sometimes so 

 numerous as to take in a large part of the cavity of the cell, so that 

 the coloured contents seem only like a deposit on its walls. Before 

 long this ' motile ' cell acquires a peculiar saccular investment, w r hich 

 seems to correspond with the cellulose envelope of the ' still ' cells, 

 but is not so firm in its consistence (I, K, L) ; and between this and 

 the surface of the ectoplasm a considerable space intervenes, tra- 

 versed by thread-like extensions of the latter, which are rendered 

 more distinct by iodine, and can be made to retract by means of 

 reagents. The flagella pass through the cellulose envelope, which 

 invests their base with a sort of sheath, and in the portion that is 

 within this sheath no movement is seen. During the active life of 

 the ' motile ' cell the vibration of these flagella is so rapid that they 

 can be recognised only by the currents they produce in the water 

 through which the cells are quickly propelled ; but when the motion 



