584 FERTILIZATION 



macrogamete (ovum). The difference in appearance between the male 

 and the female gametes, as well as between the two kinds of animals 

 which produce them, is so clearly recognizable in most cases that we have 

 come to think of sex in terms of the differences between maleness and 

 femaleness. Among the Protozoa we find some species which also show 

 a clear differentiation between male and female gametes, even though 

 the sex differences between the organisms producing them are not so 

 apparent. The protozoan organism is unicellular, and in many cases this 

 single cell produces both male and female gamete nuclei in a kind of 

 hermaphroditism (e.g., wandering and stationary pronuclei in ciliates). 

 This complication makes the homologies between Metazoa and Proto- 

 zoa less easily understandable. 



In a great many Protozoa there is no apparent differentiation between 

 gametes, yet their formation and fusion are accompanied by the same 

 fundamental processes as is the case with differentiated gametes. Isog- 

 amous reproduction, therefore, is considered a sexual process. 



The difference between the individual and the gamete is not always 

 clear in Protozoa. Perhaps the most primitive kind of sexual phenomena 

 is exemplified by two Protozoa, apparently identical to the vegetative 

 forms, coming together and fusing in a fertilization process. According 

 to Dobell (1908), this occurs in the flagellate Copromonas subtil is. 

 Nuclear "reduction" occurs after partial fusion of the cell bodies and 

 before nuclear fusion (Fig. 140). 



Copulation 

 gametic meiosis and fertilization 



Copulation, the complete and permanent fusion of gamete cells, is 

 the type of sexual activity found generally in the Plasmodroma. In cases 

 in which the parent organism gives rise to specialized cells which per- 

 form in fertilization, the process is known as fertilization by union of 

 gametes, or simply gamogamy. Both isogamy, the union of similar 

 gametes, and anisogamy or heterogamy, the union of dissimilar gametes, 

 are found in this group. In cases in which gametes are as extremely dis- 

 similar as spermatozoa and ova, the union is sometimes referred to as 

 oogamy. In cases in which the organism itself fuses with another or- 

 ganism in permanent union, the whole organism functions as a gamete 

 and the process is called hologamy. 



