564 Annals of the South African Museum. 



inches. Finally a temperature of 96 F. at noon was recorded, and the 

 rampant growth of the Volvox was cut short the following day 

 hardly any Volvox remained although water still stood in the pools 

 to a depth of 4 inches, and on the previous day both sexual and 

 asexual colonies were abundant. Two days later not a trace of Volvox 

 could be found. Whether this disappearance was due entirely to too 

 high a temperature, or to a sudden altering of the surrounding medium 

 by a shower of rain that fell on the succeeding day, or to some quite 

 different cause, is a problem still awaiting solution. Certainly during 

 this hot spell in general appearance and behaviour this Cape Flats 

 Volvox became very similar to the form of the species collected in 

 Northern Rhodesia (Rich and Pocock, p. 449). 



A similar sudden cessation apparently at the height of its develop- 

 ment has recently been observed in V. Rousseletii at Kimberley. 

 In this case, too, it occurred when the level of the water in the pool 

 fell to 4 or 5 inches. 



B. The Male Initial Cell, Antheridium Mother-cell, or Andro- 

 gonidium. 



1. V. Rousseletii. The male initial cells are found in purely male 

 colonies ; very rarely isolated ones develop in the female colonies, 

 the sperm bundles formed from them being often slightly abnormal. 

 Such globoids were not seen in the Rietfontein material, but in 

 exceptionally rich sexual material from Kimberley they are not 

 uncommon here and there in otherwise purely female colonies. 



Such occurrences are, however, exceptional, and the rule is for 

 this species to be dioecious. 



The male initial cells are scattered thickly over from two-thirds 

 to three-quarters of the surface of the coenobium, leaving free only 

 a comparatively small area at the anterior pole (Plate XX, B, D). 

 Owing to the varying rate of development it is difficult to obtain 

 accurate counts, since some have matured and the sperm bundles 

 have escaped, while others are still only at the first stage of enlarge- 

 ment. In the Rietfontein cultures, however, repeated camera-lucida 

 counts of isolated male colonies gave 150 to 200 as the average 

 number, but this should probably be rather higher. In the Kimberley 

 material, however, this number was enormously exceeded one very 

 large male colony gave a count of over 800 androgonidia in all stages, 

 from young ones still enlarging to mature sperm globoids which 

 amply justifies West's (1918, p. 2) description of " androgonidia 

 very numerous, usually several hundred." 



