212 COMPARATIVE ANATOMY 



envelope, pressed up against the envelopes of six adjacent 

 ceils, has a hexagonal outline. The bodies of adjacent cells 

 are in direct organic continuity by means of fine processes 

 of protoplasm, six of which radiate from every cell, perforate 

 the centres of the six side-walls of the envelope, and are 

 continued into the similar processes radiating from the six 

 adjacent cells. Every one of these cells has a nucleus, a con- 

 tractile vacuole, a chromatophoi, an amylum body, and a pair 

 of flagella, which last project from the surface of the colony 

 and serve, in conjunction with the similar flagella of the other 

 cells, as the locomotor organs of the colony. These ordinary 

 or somatic cells multiply by division and thereby increase 

 the number of units composing the colony, but they are 

 unable to reproduce the colony. This function is limited to 

 a small number of cells, distinguished from the somatic cells 

 by their larger size and the absence of flagella. In Volvox 

 globator eight of such cells are capable, by repeated binary 

 division, of giving rise to new colonies, and, since this mode 

 of reproduction is not preceded by conjugation, the cells in 

 question are known as parthenogonidia. Without entering 

 into the details of division, it may be stated shortly that the 

 cell-bodyof each parthenogonidium undergoes repeated division 

 within its special envelope, giving rise to a spherical colony 

 which at first projects into the central cavity of the mother 

 colony. Eventually the envelope of the parthenogonidium 

 gives way, and the young Volvox, which has by this time 

 developed flagella, falls into the cavity of the mother colony. 

 Here it remains for a while, but finally escapes by rupture 

 of the maternal wall. This method of asexual reproduction 

 is continued for several generations, after which individual 

 colonies make their appearance, differing from the normal 

 asexual colonies in that they possess some fifty large non- 

 flagellate cells which in their structure and size resemble 

 parthenogonidia. Their fate, however, is different. Some of 

 them simply increase in size, acquire a deep green colour, 

 and project into the central cavity of the colony. These 

 are the macrogametes or ova, and they are incapable of further 

 development unless they are fertilised. 



The microgametes or spermatozoa are formed from large 

 non-flagellate cells which at- first are exactly similar to those 

 which give rise to the female cells. These cells divide 



