DIMORPHISM AND POLYMORPHISM 355 



organised multicellular algae of a globular form, which rotate 

 and are propelled through the water by the movement of their 

 cilia. In addition to multiplication by means of asexual germ- 

 cells, several genera of this order, such as Eudorma and Pa7i- 

 dorina, exhibit a sexual mode of reproduction, consisting in the 

 conjugation — i.e. complete fusion — of two germ-cells which are 

 apparently quite similar to one another. 



As long as this is the case in all individuals, we might sup- 

 pose that the reproductive-cells are controlled by the idioplasm 

 which directs the development of the species in general, — that 

 is, by the germ-plasm, which is composed of a varied number of 

 similar determinants. This will, however, no longer be the case 

 as soon as the conjugating germ-cells become differentiated 

 into male and female, as has occurred in the allied genus Vol- 

 vox. The utilitarian motive for this differentiation is not far to 

 seek, for it must be advantageous for the germ-cells to contain 

 the greatest possible accumulation of nutritive material ; and 

 this could only occur to a very slight extent as long as the two 

 germ-cells destined to undergo amphimixis retained a marked 

 power of movement, as is the case in Pandorina. They con- 

 sequently become differentiated into the stationary egg-cell, 

 containing a large quantity of nutritive material, and the small 

 motile sperm-cell, in which very little food-material is present. 

 On w^hat process in the idioplasm, then, does this differentiation 

 depend ? 



The idioplasms of the egg- and sperm-cells evidently cannot 

 be precisely similar. They cannot simply consist of germ- 

 plasm ; but the egg-cell must contain a determinant which gives 

 it the histological character by which it is distinguished from 

 the sperm-cell, which latter must also contain a determinant 

 controlling its histological development. The germ-plasm of 

 Volvox must therefore contain spermatogeiietic and oogenetic 

 determinants besides those for the ciliated somatic cells, only one 

 or the other of which, however, becomes active, and impresses 

 the male or female character on the germ-cell. I imagine that 

 these sexual determinants are double, each of the two parts always 

 occurring together and being closely united, but so regulated that 

 only one of them becomes active at a time. We might repre- 

 sent this figuratively by supposing that each id of the germ-cells 

 consists of a central globular mass of germ-plasm, surrounded 

 by a layer composed of this sexual double-determinant, either 



y 



