SEXUAL DIFFERENTIATION IN PROTOPHYTA * 91 



^ 

 individuals. They are shown imbedded in the gelatinous 



matrix of the large colony on the left of the figure. The micro- 

 gametes or spermatozoa, 1 on the other hand, are much smaller, 

 club-shaped bodies, having a characteristic yellowish colour and 

 with a pair of flagella at the narrow, pointed end (Fig. 40, M 3 ). 

 They are produced in bundles of sixty-four by repeated divi- 

 sion of a mother cell (Fig. 40, II VI). Thus the male and 

 female gametes, spermatozoa and ova, do not occur together 

 in the same colony, but the colonies, when they consist of 

 gametes and not of ordinary individuals, contain only one or the 

 other kind. Hence the sexual differentiation is in this case 

 extended from the gametes themselves to the colonies which 

 bear them, and we may recognize colonies of three kinds : (1) 

 Asexual, which produce no gametes and reproduce by ordinary 

 fission of all or any of the component cells, (2) male sexual, which 

 produce microgametes or spermatozoa, and (3) female sexual, 

 which produce megagametes or ova. The bundles or colonies of 

 microgametes (Fig. 40, MI, Mg) swim about actively by means of 

 their flagella, apparently in search of the larger and much less 

 active female colonies (Fig. 40, I). Having found such a 

 colony the now separated microgametes (Fig. 40, M 3 ) bore 

 tlieir way in, ultimately conjugating with the megagametes to 

 form zygotes. 



In any one colony of Pandorina or Eudorina the constituent 

 ^ells are all of one kind, all ordinary asexual cells or all male 

 gametes or all female gametes. In Volvox (Fig. 11) we meet with 

 a further advance. Even in asexual colonies which do not produce 

 gametes at all we find the cells differentiated in so far that some 

 only are capable of giving rise (asexually) to daughter colonies, 

 while in the female colonies only some of the cells form female 

 gametes or ova (Fig. 11, A, o). The male gametes (Fig. 11, B D) 

 are very similar to those of Eudorina. They unite with the 

 female gametes to form zygotes, which, after a period of rest, 

 develop into new Volvox colonies. 



In order to emphasize the fact that the process of conjugation 

 is essentially a nuclear phenomenon we may now turn to the 

 case of Paramoecium. The general appearance and structure of 

 this protozoon have been described in Chapter IV. (Fig. 8). It 

 multiplies by simple transverse fission, and under favourable 

 conditions continues to do so until exhaustion sets in, when its 



1 Often termed by ^tanists spermatozoids. 



