532 Annals of the South African Museum. 



number 180 (Plate XX, B-F). To these must be added occasional 

 ' mixed " colonies containing both asexual daughter colonies and 

 either sperm globoids or oospores (Plate XXXVIII, B, C). Such 

 colonies were comparatively rare in the Kietfontein material, but 

 less so in that from Kimberley. Even rarer were female colonies 

 with occasional sperm globoids, often somewhat abnormal, which 

 were very occasionally seen in the latter material. The anterior pole 

 is always free of reproductive cells, these being as a rule confined to 

 the posterior half in the case of the asexual gonidia, to the posterior 

 two-thirds or three-quarters in the case of the sexual cells. 



At the height of its growth, under favourable conditions it is one 

 of the largest forms of Volvox yet recorded, occasionally reaching a 

 diameter of over 2 mm., though this is exceptional. In a typical 

 healthy, well-grown strain, however, the mature asexual colonies 

 are mostly over 1-5 mm. in equatorial diameter. The sexual colonies 

 are in general smaller than the asexual, but even they are often 

 over 1 mm. in diameter. The only recorded forms which exceed it 

 in size are V. (Besseyosphaera) Powersii (Shaw), Printz (Shaw, 1916, 

 p. 253), 1-8-2-5 mm. diameter, and the recently discovered "Kim- 

 berley Giant," V. gigas, which may reach the huge size of 3 mm. or 

 over ; both these species belong to different sections of the genus. 

 In the Eu-Volvox section the only form which is possibly at times 

 larger is one from S.W. Africa, V. amboensis, Eich and Pocock (loc. cit., 

 p. 462). 



West (1918, p. 2) remarks on the marked ovoid shape character- 

 istic of the mature sexual colonies as opposed to the " globose " form 

 of the asexual, but all the latter shown in his photographs are still 

 young. When mature, the asexual colonies, too, may be ovoid, 

 though in general less elongated than the sexual individuals (Plate 

 XVIII, B-D) ; the external form, however, varies greatly, being 

 influenced markedly by external conditions. 



The number of cells composing the colony is large, usually 20,000 

 to 30,000, though as many as 50,000 were estimated in some cases 

 by Eousselet (1914, p. 393) and West (1910, p. 101); the proto- 

 plasts themselves are small, with well-marked protoplasmic connec- 

 tions, larger and more widely spaced at the anterior than at the 

 posterior pole. 



Reproductive cells are differentiated by size before birth, but 

 undergo no divisions until some time subsequent to birth. The 

 sperms are formed in more or less completely closed globoids, never 

 platelets, while the oospheres ripen into spiny-coated oospores. 



