454 Annals of the South African Museum. 



15,000 to 42,000 (average 20,000 to 23,000). The mature asexual 

 colonies in a vigorous strain (Plate XX, fig. A) range from 690 x 754 /z 

 to 2058 x 2100 p. ; very large colonies were, however, rare, and were 

 only seen during the early stages of the original culture ; but at the 

 height of development of any one culture colonies about 1300 x 1500 /JL 

 or even larger were common. 



The number of daughter colonies is most often 8, but varies from 

 4 to 16 (see Plate XVIII, fig. A). The 8 primary gonidia are arranged 

 alternately in two planes, one approximately equatorial, the other 

 half-way between equator and posterior pole. The gonidia are 

 differentiated before birth probably either before or immediately 

 on completion of cell-division (Plate XVIII, fig. E) ; they are at any 

 rate distinguishable from the somatic cells very soon after inversion 

 of the daughter colony by their greater size and by the possession of 

 more than one pyrenoid. They do not begin to divide until after 

 birth. At birth their diameter is about double that of the somatic 

 cells (gonidia 11 to 12 /x, somatic protoplasts 4 to 5 /JL). The daughters 

 at birth are usually large (up to 473 x 539 /JL), but there is a wide range 

 in size. The smallest free colony measured was 182 x 204 /M, while the 

 largest in situ was 630 x 672 /x, but this was exceptional. 



The number of daughters depends partly on the phase reached, 

 partly on external conditions, mainly temperature and supply of 

 nutritive material. The groups shown in the microphotographs 

 (Plate XVIII, figs. A-D) illustrate excellently the characters seen in 

 successive phases (A, C, and D were from one culture, B from a 

 later one, as the corresponding phase had been missed in the earlier 

 series) : 



A. 22nd February 1930. Mr. Steer's original culture at the height 

 of the first asexual phase. Daughter colonies up to 13 in number. 

 Colonies large at about this time the largest colonies (over 2 mm. in 

 diameter) were seen. 



B. llth January 1931. From a culture derived from the above : 

 sexual phase at its height, male colonies large, the majority of the 

 sperm globoids having already escaped ; female colonies with ripening 

 oospores predominating. Asexual colonies with few daughters. 



C. 23rd March 1930. Original culture end of sexual phase ; male 

 colonies very few, with scattered male globoids. Asexual colonies 

 with 2 to 5 daughters. 



D. 28th March 1930. Second asexual phase. Daughter colonies 

 very few 1 to 3, most often 2. Soon after this date Volvox practi- 

 cally disappeared from the culture for some weeks. 



