178 



Volvocineae 



the former the cells are so far removed from each other that the whole colony 

 appears as a ' sponge-like reticulum,' whereas in the latter they are so com- 

 pactly arranged that the whole coenobium has a very firm and solid aspect. 

 The coenobium of Volvox, like that of Pleodorina, has a definite polarity, and 

 Overton ('89) has pointed out that in V. aureus the stigma (' eyespot ') of 

 each cell lies on the side turned towards the anterior pole, probably in relation 

 to the positive phototaxy of the coenobium. In Pandonna and Eudorina 

 the stigma is large in the anterior cells, smaller in the equatorial cells and 

 invisible in the posterior cells (Conrad, '13). 



In the lower forms of the Volvocese, Gonium, Pandorina and Eudorina, 

 reproduction takes place by the formation of a daughter-coanobium from 

 every cell of the mother-colony (figs. 103 F and 104 B). The daughter- 

 coanobium is formed within the wall of the mother-cell, which swells up 

 and becomes mucilaginous, and ultimately by its dissolution sets the young 

 colony free. In the Highest type, namely Volvox, only certain of the re- 

 productive cells, often termed parti henoyonidia, give rise to daughtcr-coenobia. 



- vc 



Fig. 106. Pleodorina illinoisensis Kofoid. A specimen from Madras, x 500. gc, gonidial or 

 reproductive cell; vc, vegetative cell. The arrow marks the direction of translocation. 



An intermediate condition is found in Pleodorina calif ornica, in which the 

 formation of daughter-colonies is restricted to certain purely reproductive 

 cells located in one half of the colony ; and in PI. illinoisensis there is a 

 similar but sometimes a much less obvious restriction of the reproductive 

 cells. In Volvox the parthenogonidia vary from 1 to 24 (commonly 4 or 8) 

 and are scattered irregularly among the somatic cells. In Platydorina the 

 young daughter-colony is at first cup-shaped, but subsequently becomes 

 flattened and twisted. The normal development in Volvox begins with 

 the enlargement of the mother-cell, followed by successive divisions into 



