PEDIASTEE.E 567 



such a manner that the resultant product may vary greatly in the 

 number of its cells, and consequently both in size and in form. 

 Thus in Pediastrum granulation (fig. 428) the zoospores formed by 

 the subdivision of the endochrome of one cell, which may be four, 

 eight, sixteen, thirty-two, or sixty-four in number, escape from the 

 parent-frond still enclosed in the inner layer of the cell-wall ; and it 

 is within this that they develop themselves into a cluster resembling 

 that in which they originated, so that the frond may be composed of 

 either of the just-mentioned multiples or sub-multiples of 16. At 

 A is seen an old disc, of irregular shape, nearly emptied by the 

 emission of its zoospores, which had been seen to take place within 

 a few hours previously from the cells a, b, c, d, e ; most of the 

 empty cells exhibit the cross slit through which their contents had 

 been discharged ; and where this does not present itself on the side 

 next the observer, it is found on the other. Three of the cells still 

 possess their coloured contents, but in different conditions. One of 

 them exhibits an early stage of the subdivision, of the endochrome 

 namely, into two halves, one of which already appears halved again. 

 Two others are filled by sixteen very closely crowded zoospores, only 

 half of which are visible, as they form a double layer. Besides 

 these, one cell is in the very act of discharging its zousport-s. nine of 

 which have passed forth from its cavity, though still enveloped in a 

 vesicle formed by the extension of its innermost membrane ; whilst 

 seven yet remain in its interior. The new-born family, as it 

 appears immediately on its complete emission, is shown at B ; the 

 zoospores are actively moving within the vesicle, and they do not as 

 yet show any indication either of symmetrical arrangement or of 

 the peculiar form which they are subsequently to assume. Within 

 a quarter of an hour, however, the zoospores are observed to settle 

 down into one plane, and to assume some kind of regular arrange- 

 ment, most commonly that seen at C. in which there is a single 

 central body surrounded by a circle of five, and this again by a 

 circle of ten ; they do not, however, as yet adhere firmly together. 

 The zoospores now begin to develop themselves into new cells, 

 increase in size, and come into closer approximation (D) ; and the 

 edge of each, especially in the marginal row, presents a notch which 

 foreshadows the production of its characteristic 'horns.' Within 

 about four or five hours after the escape of the zoospores, the cluster 

 has come to assume much more of the distinctive aspect of tin- 

 species, the marginal cells having grown out into horns (E) ; still, 

 however, they are not very closely connected with each other, and 

 between the cells of the inner row considerable spaces yet intervene. 

 It is in the coiirse of the second day that the cells become closely 

 applied to each other, and that the growth of the horns is completed, 

 so as to constitute a perfect disc like that seen at F, in which, how- 

 ever, the arrangement of the interior cells does not follow the 

 typical plan. 1 The formation, of ' microzoospores ' has also been 

 observed, which have been seen to conjugate. 



1 See Prof. Braun on The Phenomenon of Rejuvenescence in Nature, published 

 by the Ray Society in 1853 ; and its subsequent memoir, --!/.'/ rum Unicellular/nit 

 Genera nova aut minus cognita, lH5-">. 



