FERTILIZATION 199 



the megagamete. In other forms the microgametes are considerably 

 modified structurally, usually in connection with an increase in loco- 

 motor activity. At the same time the megagamete may increase con- 

 siderably beyond the ordinary vegetative size and may then lose motility 

 more or less completely. So it finally comes about that gametes of two 

 wholly different types are formed, both quite unlike vegetative cells, 

 and the typical Metazoan condition is reached. 



Several groups of Protozoa, e.g., Gregarines, colonial Flagellates, 

 afford interesting series showing stages in this differentiation of the 

 gametes (Fig. 104). We may outline one such series selecting examples 

 from the Volvocine group of Flagellates. 



In Stephanosphcera all the individuals of the colony are, or may be, 

 reproductive, and conjugation is isogamous and endogamous. There 

 is no differentiation of gametes. In Pandorina (Fig. 8) all of the indi- 

 viduals may be reproductive, but some of the gametes may be differ- 

 entiated in size, and conjugation may be either isogamous or anisogamous 

 as described above. Here dissimilarity of the gametes is facultative. 

 In Eudorina two kinds of colonies are found. In one, all the cells may 

 become reproductive, the individuals forming megagametes only slightly 

 larger than vegetative cells ; in another only four cells of the colony are 

 reproductive and each of these forms sixty-four very small and active 

 microgametes. Fertilization is here strictly anisogamous and exo- 

 gamous. The last step is represented. by Volvox (Fig. 10), where the 

 number of gamete forming cells is always limited. Here too differentia- 

 tion of the gametes reaches its climax among the Protozoa, and the 

 Metazoan condition is reached. The reproductive cells lose their motor 

 organs and begin to enlarge. A few of them grow to a relatively 

 enormous size and become the passive megagametes. The others 

 grow to lesser extent and then divide rapidly, each forming, probably 

 128 microgametes. These are very small flagellated, extremely active 

 cells, with an elongated rostrum or penetrating organ at one end (Fig. 

 104, d). The microgametes are liberated in large numbers and swim 

 about until one reaches a megagamete which it then enters and their 

 nuclei fuse forming a typical zygote which then reproduces a new 

 colony. The resemblance to the gametes of the Metazoa is so com- 

 plete that they are here termed the oosphere, or ovum, or oogamete 

 (megagamete) and the spermatozoon or spermagamete (microgamete) . 



It should perhaps be noted here that the process of conjugation or 

 fertilization is not always associated with the reproduction of the 

 Protozoa mentioned above, not even in the colonial forms. For the 

 usual reproductive processes are carried out by the simple fission of 

 ordinary vegetative cells. In the simpler colonial forms, such as 

 Pandorina and Eudorina, as many new colonies may be formed as there 

 are individuals forming the original colony, in Volvox, however, the 



