Observations on the Genus Volvox in Africa. 457 



pyrenoid, central elongated nucleus, red eyespot, and 2 contractile 

 vacuoles near the anterior end, below the base of the elongated hyaline 

 beak. At the apex of the beak are inserted the two long backwardly 

 directed cilia (up to 23 p. in length). The spermatozoids swim 

 actively in the water, or they may be seen inside the coenobium, where 

 they move in an amoeboid manner, the cilia remaining motionless. 



The female coenobia are usually slightly larger than the male ; 

 they produce from 60 to over 200 oogonia, each of which develops 

 a single large flask-shaped oosphere (Plate XXI, fig. D), the apex of 

 which lies a short way below the surface of the colony and is con- 

 nected by protoplasmic strands with neighbouring cells. Later, pre- 

 sumably after fertilisation, the oospore rounds off and lies entirely 

 within the peripheral zone of the parent (Plate XXI, fig. E). In the 

 young colony shown in optical section in Plate XX, fig. D, most of the 

 oospores have sunk inwards, but a few flask-shaped oospheres are still 

 visible round the wall. The oospore develops as in V. capensis ; the 

 shape of the conical spines, however, is different they are longer, 

 slightly narrower, and usually more numerous in an optical section 

 (Plate XXI, fig. C, and text-fig. 4, B). The size of the oospore, 

 including spines, is about the same as in F. capensis ; hence since the 

 spines are longer, the spore body in V. Rousseletii is actually 

 slightly smaller. Plate XX, figs. E and F, show two large female 

 colonies with nearly mature oospores. The absence of gaps in the 

 cell network is noticeable. 



Occasionally mixed colonies containing asexual daughters and 

 either sperm globoids (Plate XX, fig. C) or oospores are seen. Accord- 

 ing to our experience these mixed colonies may occur in any of our 

 forms as the strain advances in age, hence usually towards the end 

 of the sexual phase, never in the early stages of development. 



In V. Rousseletii as a rule the sexes are quite distinct, but very rarely 

 a male globoid occurs in an otherwise purely female coenobium. Such 

 cases occurred so rarely during the time (18 months) that the Riet- 

 fontein cultures were under observation, that they may be regarded 

 as exceptions to the general rule. 



Dimensions of V. Rousseletii. (Mr. Steer's Rietfontein culture.) 



Mature asexual colony, 690 x 754 p to 2058 x 2100 p. 



Mature sexual colony, male, 646 x 753 p, to 1184 x 1378 p. ; average 



700 x 770 ii. 

 Mature sexual colony, female, 646 x 711 p, to 1058 x 1292 p, ; average 



840 x 960 p.. 



