Volvox in South Africa. 617 



(the endospore) and lying more or less eccentrically in the hyaline, 

 rather thick mesospore from which it is separated by a clear empty 

 zone. The escaped spore is 31 to 35 ^ in diameter, the brown content 

 27 to 31 /i, (Plate XL VI, E). 



Stage 2. The spore content increases in size until it completely 

 fills the mesospore (Plate XLVI, F) ; the contrast between the 

 appearance at the beginning and end of this stage is well shown in 

 the photograph. The size of the whole increases considerably. 

 These two stages may sometimes be reversed, the enlargement taking 

 place before escape from the spiny epispore (Plate XLVI, D), but in 

 the majority of cases seen development proceeds in the order given. 

 It is at this stage that the oospores are often consumed by para- 

 moecium, worms, etc. ; the comparatively thin-walled, immobile, 

 spherical structures seem peculiarly vulnerable. 



Probably during this period of enlargement reduction division takes 

 place. In life, division of the nucleus has not as yet been detected, 

 although spores have been seen with their contents divided into four. 

 The spore now measures 35 to 40 /x. 



Stage 3. The mesospore splits and the delicate endospore pro- 

 trudes, enlarging to form a vesicle into which the content of the spore 

 passes. Usually this already shows the form of a large zoospore 

 with clear, bluntly pointed anterior end (Plate XLVI, G) ; sometimes, 

 however, more than one zoospore appears to be formed (Plate XLVI, 

 H). This point has not as yet been cleared up, and it is possible 

 that not more than one zoospore survives, although the appearance 

 in the few such cases seen does not suggest this, as, for example, in 

 the spore shown in this figure, where it certainly looks as if four 

 equal sized zoospores were being formed. Such structures were far 

 less common in the Rietfontein material than those of the type shown 

 in Plate XLVI, G, and, with one possible exception, in these no traces 

 of vestigial zoospores such as are found in Eudorina, etc. were seen. 



Stage 4. Escape of Zoospore. The zoospore escapes from the endo- 

 spore vesicle into the water, two cilia are emitted, and a delicate loose 

 wall is formed at first close to the zoospore (Plate XLVI, I), later 

 enlarging, and the zoospore begins to move about in the water, at 

 first slowly, later with some rapidity (Plate XLVI, J). 



Structure of the Zoospore. 



The zoospore is a large Chlamydomonas-like structure, broadly 

 ellipsoidal but slightly asymmetric in form, of a deep golden-brown 

 colour, except at the clear bluntly pointed end and here and there 

 VOL. xvi, PART 3. 40 



