194 



GENERAL EMBRYOLOGY 



through the fact that reduction in size is usually the result of multiple 

 fission or brood formation, which increases the number as well as the 

 activity of the gametes. In all of these forms of isogamy the union 

 of the gametes is permanent, the conjugants fusing completely and 

 thereby losing their identity as individuals. It need hardly be added 

 that in these cell conjugations the essential step seems to be the fusion 

 of the gametic nuclei into a single zygote nucleus. 



FIG. 101. Gamete formation and fusion (isogamy) in the Flagellate, Chlamy- 

 domonas steinii. After Goroschankin. A. Vegetative form, n, nucleus. 

 B. Group of gametes formed by multiple fission. C. Single gamete. D, E, F. 

 Stages in the fusion of gametes (isogametes). G. Zygote. 



A special form of isogamy needs particular notice on account of its 

 frequency among the most familiar Protozoa the Ciliata. This is a 

 temporary form of isogamy which involves, not the fusion of two gametes 

 to form a zygote, but the mutual fertilization of the two gametes through 

 the exchange of nuclear substance and perhaps also a small amount of 

 cytoplasm. The details of nuclear behavior in the conjugation of Para- 

 moecium, for example, are probably familiar but will bear brief restate- 

 ment here. This outline refers particularly to that form of Paramcecium 

 having only a single micronucleus ; one should recall that in these forms 

 which have both micronucleus and macronucleus, the former is, or 

 represents, the idiochromidia, the latter the vegetative nuclear struc- 

 tures. Two Paramoecia of normal vegetative size and external form 

 meet side by side, oral surfaces in contact, in a sort of plastogamic union, 

 The further course of events is exactly similar in each individual of 



