Volvox in South Africa. 625 



short. The whole appearance except for the colour is very like that 

 of a sperm globoid at the same stage, but the common wall soon 

 becomes clearly apparent outside the whole colony (Plate XL VII, L). 

 Enlargement, still within the vesicle, follows ; the protoplasts in- 

 crease in size and become more distant, and the cilia lengthen till 

 the whole vesicle is filled (fig. 10, J). 



The vesicle finally disappears. The young colony was never actually 

 seen escaping, and there is some evidence that the vesicle wall dis- 

 integrates in one case, while the vesicle was intact in parts, in others 

 the cilia were protruding through it ; possibly here, too, there is 

 enzyme action on the part of the young colony, or possibly the action 

 of the latter in its now fairly active movement is sufficient to destroy 

 the aged and much-stretched vesicle wall. Colonies as large as 53 /z 

 and 66 JJL were seen still within vesicles which measured 62 JJL and 75 /JL 

 respectively. In the first of these the cilia were sweeping the inside 

 of the vesicle and some protruding from it where the wall had been 

 destroyed. 



Meanwhile the colour of the protoplasts has been growing lighter. 

 The haematochrome is gradually replaced by chlorophyll, remaining 

 here and there as red globules, sometimes collected near the centre, 

 sometimes near the apex of the cells, and finally disappearing entirely 

 except in the large eyespots, which now for the first time become visible 

 in the anterior cells (Plate XL VIII, B, C). 



This colour-change begins while the colony is still enclosed in the 

 vesicle, but is not completed until after it has been moving freely 

 for some time. With the differentiation of the eyespots the colony 

 first becomes phototactic. 



II. THE JUVENILE FORM. 



The young colony is now growing into the very characteristic 

 Juvenile form, a small structure sharply distinguished from the Adult 

 form the typical Volvox colony by its much smaller size and few, 

 considerably larger, cells. 



The vegetative cells are essentially similar in structure to those 

 of the adult, but the protoplasts are larger and more widely spaced 

 with very strong thick connecting strands, the two cilia are inserted 

 wide apart, and the limiting membranes are very distinct (fig. 10, K). 

 The inner and outer wall of each cell forms a well-marked curve, 

 so that both inner and outer membranes, instead of appearing each 

 as a more or less continuous curve, are built up of a series of separate 



