Sph&reHaeese 167 



cell-sap vacuoles in the interior of the cell, such vacuoles gradually extending 

 outwards and so breaking down the chloroplast at various points. In Stepha- 

 nosph&m there are only two pyrenoids in the chloroplast (fig. 96 G and H), 

 but in Sph&rella there are from two to eight. There are numerous contractile 

 vacuoles in the peripheral part of the protoplast, Wollenweber recording as 

 many as 60 for Sph. Droebakensis. In both genera the bright red pigment 

 haematochrome is prominent, and in Sphaerella both motile and resting-cells 

 may be so completely tinged with red that the cytological characters of the 

 living cell are entirely obscured. The hsematochrome usually makes its 

 first appearance in that part of the protoplast immediately surrounding 

 the nucleus. 



The motile vegetative cells (= zoogonidia) of Sphterella readily assume the 

 condition of spherical resting-cells, which are completely red, 10 70 /* in 

 diameter, and surrounded by a thick cellulose wall (fig. 96 F). All structural 

 details are hidden by the hsematochrome, which according to Zopf ('95), is 

 partly in solution in oil-drops and partly in a microcrystalline form. Divisions 

 within the resting-cell result in the formation of 4, 8 or 16 daughter-cells. 

 These become, under favourable conditions, vegetative motile cells (or mega- 

 zoogonidia) which grow much in size while swimming about. If the 

 conditions are unfavourable they remain as further resting-cells, so that it is 

 possible for the resting-cells to increase greatly in numbers and gradually form 

 a stratum. The motile cells may divide repeatedly to form further megazoo- 

 gonidia or they may return to the resting-state. They may also form micro- 

 zoogonidia (identical with the megazoogonidia except in size), 32 or 64 being 

 formed in each cell. Some confusion appears to have existed in the past 

 between the microzoogonidia and gametes; the former frequently die, or 

 they may come to rest, form a cell-wall, and grow into normal resting-cells. 

 Both Wollenweber ('08) and Peebles ('09) have observed isogametes (fig. 

 96 E), as many as 100 being formed in one gametangium. Conjugation of 

 the gametes to form zygotes has also been observed. 



In Stepkanosph&ra, in which the coenobium normally consists of eight 

 cells arranged in an equatorial zone within a tough spherical or ellipsoidal 

 investment, multiplication occurs by the division of each cell of the coenobium, 

 after having assumed a more or less globular form, into 4 or 8 daughter-cells, 

 each group forming a new coenobium. Gamogenesis occurs by the conjugation 

 of isogametes, of which 8, 16, or 32 are formed within the mother-cell. As 

 in Sphterella, the gametes are fusiform in shape and they fuse laterally to 

 form spherical ' zygozoospores/ which soon become quiescent and turn yellow- 

 brown in colour. 



Wollenweber ('08) has shown that by appropriate methods of culture, Sphxrella may 

 be made to pass into any of its different states, yielding zoogonidia, zygotes, aplano- 

 spores, or palmella-state. He also showed ( J 07) that n piginont-spot (or sti-inn) 



