864 LIFE : OUTLINES OF GENERAL BIOLOGY 



namely the separation between special reproductive cells or germ- 

 cells and the ordinary somatic cells or body-cells. 



A third step was the distinction between individuals producing 

 ova and others of the same kind producing spermatozoa. This was 

 the beginning of sex in the strict sense ; and it is well illustrated by 

 Volvox, which we regard as a colony-forming Infusorian, though 

 it is sometimes claimed by the botanists as a free-swimming fresh- 

 water Alga. It is a beautiful gyrating hollow ball of green biflagellate 

 cells, connected by fine protoplasmic bridges and embedded in a 

 gelatinous matrix, from which their flageUa project. In Volvox 

 globator the average number of individual units is about 10,000; 

 in V. aureus or minor, 500-1,000. Each cell contains a nucleus, a 

 contractile vacuole, and chlorophyll corpuscles, which enable 

 Volvox to feed holophytically, i.e like a green plant. At the anterior 

 hyaline end of each cell, where the two flagella are inserted, there is 

 a pigment spot, which has probably some orientating function. 



In its method of reproduction Volvox is of much biological interest 

 and importance. As Klein, one of its best describers, says, it is an 

 epitome of the evolution of sex. Some of the colonies are asexual. 

 In these a limited number of cells possess the power of dividing up 

 to form Uttle clusters of cells; these clusters escape from the envelope 

 of the parent colony, and form new free-swimming colonies. In other 

 colonies there are special reproductive cells, which may be called 

 ova and spermatozoa (Figs. 147-149). 



In V. globator the two kinds of reproductive cells are usually 

 formed in the same colony, the formation of spermatozoa generally 

 preceding that of the ova. Technically the colony may then be 

 described as a protandrous hermaphrodite. 



In V. aureus the colony is oftenest unisexual or dioecious, i.e. either 

 male or female. But it is sometimes monoecious or hermaphrodite, 

 and in this case it is generally protogynous, i.e. producing eggs first. 



Whether in a hermaphrodite or in a unisexual colony, the sex 

 cells appear among the ordinary vegetative units; the ova are 

 distinguishable by their larger size, the "sperm mother-cells" divide 

 rapidly and form numerous (32-100 or more) slender spermatozoa, 

 each with two cilia. In V. globator their bundles may break up within 

 the parent colony; or, as always occurs in V. aureus, they may 

 escape intact, and swim about in the water. In any case, an ovum is 

 fertilised by a spermatozoon, and, after a period of encystation and 

 rest, segments to form a new colony. Occasionally, however, this 

 organism, so remarkable a condensation of reproductive possibihties, 

 may produce ova which develop parthenogenetically. 



Here, then, we have an organism, on the border-line between 

 plant and animal life, just across the line which separates the 

 unicellular from the multicellular, illustrating the beginning of that 

 important distinction between somatic or body cells and reproductive 



