I VOLVOX 41 



The microgametocytes are found in very young colonies and may 

 form a large proportion of all the cell-individuals. Each microgametocyte 

 divides repeatedly by fission, forming eventually a slightly curved disc 

 composed of numerous elongated cells — ^the microgametes. Each micro- 

 gamete (Fig. 17, D) possesses the same details of structure as the ordinary 

 individual of the colony only the proportion and arrangement of these 

 are different. In shape it is much more slender, one end being drawn 

 out to a fine point and the other rounded. The pointed end is prolonged 

 into the two flagella which are long and powerful. The nucleus is 

 elongated, the stigma well developed, and the chromatophore is situated 

 at the rounded end. When the flagella of the microgametes become 

 active the whole mass is moved about and it may make its way out of 

 the colony. Sooner or later however the microgametes separate and 

 swim away through the water as independent cells. If one of these 

 comes into the neighbourhood of a colony containing a macrogamete it 

 bores its way in and syngamy takes place. The zygote immediately 

 proceeds to surround itself with a clear transparent cyst or shell and the 

 presence of this affords a conspicuous character by which the zygote 

 can be at once recognized and distinguished from a macrogamete. When 

 the colony as a whole dies and disintegrates the zygotes fall down into 

 the mud and remain there within their protective shell or envelope until 

 conditions are favourable for their development into adult colonies. 



The study of Volvox introduces us to several matters of general 

 biological importance. While the Volvox consists of ordinary cell- 

 individuals these are not independent but are linked together into a 

 community which constitutes itself an individual of a higher order. 

 And the cells which form this individual are sharply differentiated into 

 two sets — the ordinary cells and the reproductive cells. The importance 

 of this lies in the fact that here for the first time we find an arrangement 

 that is universal among the higher animals, the body of which invariably 

 consists of a great mass of non-reproductive cells known as the soma, and 

 a special set of purely reproductive cells known as the gonad (see p. 85). 



We see also in Volvox a differentiation of two sexes. This consists 

 fundamentally of a specialization of the gametes in two different directions. 

 Essential features of the gametes are (i) that they shall have the power 

 of uniting in syngamy and (2) that the resulting zygote shall possess, 

 stored up in its cytoplasm, a sufficiency of yolk or food-material to start 

 the new colony on its way. To facilitate the achievement of the first' 

 the gametes are differentiated into (i) a stationary type without flagella, 

 and (2) a highly active type with greatly developed flagella which is 



