620 Annals of the South African Museum. 



Development of the Zoospore. 



(a) Period of Cell-Division. The duration of the life of the zoospore 

 as a unicellular organism is not long ; soon cell-division commences 

 and continues just as in the case of the male initial cell, except that 

 the zoospore retains its cilia (cf. development of daughter colony in 

 Eudorina, Pascher, 1927, p. 48), and continues motile until several 

 successive divisions have taken place. 



Apparently here, too, there is slight shifting of the polar axis of the 

 cell ; the protoplast is slightly oblique from the beginning, as the 

 photographs suggest, and this obliquity appears to increase before 

 division begins. In any case, the plane of the first division is not 

 exactly longitudinal to the external form of the zoospore but slightly 

 oblique, so that it does not pass through the beak. Hence one of the 

 resultant cells retains the beak with the two cilia, the other cell is 

 non-ciliate (fig. 9, C, D). Often the beak gets drawn out much as 

 do the apical portions of cells near the posterior pole of an adult 

 colony, so that the cilia are attached by a hammer-shaped proto- 

 plasmic protuberance to one of the cells derived from the zoospore 

 (fig. 9, D to F). In some zoospores the first plane of division is 

 uneven and the cilia-bearing cell is the smaller ; when its sister cell 

 next divides it remains undivided (fig. 9, E, F). In others the two 

 first cells are apparently equal and divide again simultaneously, e.g. 

 in the one whose development is shown in fig. 9, G to L, but here 

 there are indications that in later divisions the cilia-bearing cell does 

 not divide at the same rate as the others and is rather larger. The 

 period during which movement continued varied considerably in one 

 case the developing colony moved actively right up to the 32-celled 

 stage, but ceased to move before the next division was complete 

 (fig. 9, K). In another, movement ceased after the second division and 

 before the third was complete (i.e. at the end of the 4-celled stage). 

 Probably the period is determined by the rate of division of the cilia - 

 bearing cell. There must be a minimum size below which the cell is 

 no longer capable of working the long cilia ; hence, if the cilia-bearing 

 cell remains larger than its sister-cells, the period may be prolonged, 

 whereas if it divides at the same rate its size rapidly decreases and 

 movement ceases correspondingly early. The cilia may remain 

 attached to the cell for a time, but eventually become disconnected ; 

 even when the young colony is fully formed and rotating within the 

 vesicle the cilia may still be seen attached to the latter, often " frozen ' : 

 in a spiral twist. This was the case in the young colony which was 



