Volvox in South Africa. 603 



has a narrower, more elongated neck than in V. capensis (Plate 

 XVI, D), but this is not always the case. The pyrenoids become 

 very numerous and the whole structure so dense that it is only by 

 very careful focussing, and then by no means always, that it is 

 possible to distinguish the large central nucleus. A small area at 

 the apex remains clear, and in this it is sometimes possible to see one 

 or two small contractile vacuoles. The cilia are retained well into 

 the period of enlargement; when they are eventually lost the two 

 basal portions are distinguishable for some time between the outer 

 membrane and the apex of the oosphere (fig. 8, B). The protoplasmic 

 connections between the oosphere and the neighbouring cells are 

 usually attached to the upper angle of the neck. 



2. Structure of the Mature Oosphere. When mature, i.e. ready for 

 fertilisation, the oosphere is a massive, flask-shaped body, the neck 

 of the flask projecting between the neighbouring cells to within a 

 short distance of the outer membrane, the enlarged base protruding 

 into the hollow of the parent, enclosed in the delicate vesicle formed 

 by the much enlarged wall of the mother-cell. The bulk of the 

 oosphere is intensely dark green, the large central nucleus almost 

 enclosed in the massive, bowl-shaped chloroplast, very rich in chloro- 

 phyll and containing a large number of pyrenoids ; the nucleolus 

 is large and conspicuous. There is a large reserve of food, chiefly 

 starch and fatty oils. The apex remains clear and has all the appear- 

 ance of a receptive spot. In addition to the apical contractile 

 vacuoles, others in the basal part may sometimes be seen, but such 

 vacuoles in a healthy oosphere are comparatively small and rare; 

 much vacuolation is usually a pathological sign. In surface view the 

 oosphere is circular and about 30 [j, in diameter (Plate XLVI, A). 



Homologies of the Reproductive Cells. 



From the foregoing it is clear that the three types of reproductive 

 cells, the gonidium and the sexual initial cells, are similar in origin, 

 in structure, and in the first stages of development, but very early 

 in the latter divergences appear. In the case of the gonidium and the 

 male initial cell, however, development continues on similar lines 

 right up to the formation of the sperm globoid which is essentially 

 similar in structure to the asexual colony but smaller, resulting 

 from fewer successive cell-divisions ; as in the embryo colony, inver- 

 sion is followed by the secretion of a common envelope, formed 

 from the fused outer portions of the cell-walls. Whether here, too, 

 these walls are continuous round the protoplasts could not be deter- 



