150 CONJUGATION, MATURATION, AND FERTILIZATION 



C. FERTILIZATION BY EXOGAMY. 



It is not at all improbable that some of the cases that have been 

 described as autogamous may be in reality exogamous. In the multi- 

 nucleate forms, in order to decide such a matter it is necessary not only 

 to observe the union two by two of such nuclei, but their mode of 

 origin must also be known. Thus, in the mycetozoa the plasmodium 

 from which the sexual nuclei are generated is formed by the fusion of 

 two or more ameboid cells at an early period of development, hence 

 the nuclei which later fuse may be derived from different ancestral 

 cells, and such fusions w^ould not be examples of autogamy, but of 

 exogamy. In some cases of sexual reproduction among myxosporidia 

 (notably in the actinomyxidse and possibly in Spheromyxa labrazesi) a 

 similar derivation of the conjugating nuclei has been suspected. Such 

 cases of possible exogamy are well illustrated in almost any of the 

 higher types of mycetozoa, and one such has been well described by 

 Kriinzlin ('07) for Arcyria clnerea and Trichia fallax, and by Olive 

 ('07) and Jahn ('07) for Ceratiomijxa hydnoides. Without going into 

 the details the process may be summarized shortly as follows: The 

 young ameboid or flagellated spores, after assumption of the ameboid 

 state, fuse into plasmodia of considerable size. Cell boundaries are 

 entirely absent and the nuclei have an opportunity to become thor- 

 oughly mixed in the protoplasmic streaming. Fructification ensues 

 after a longer or shorter vegetative life and in these fruiting bodies, 

 or before their formation, the nuclei unite in pairs, the union being 

 followed by synapsis and double divisions and formation of the ripe 

 spores. 



A somewhat similar union has been described by Hartmann and 

 Nagler in the case of Ameba diploidea, H. and N., where the organism 

 is binucleated throughout the ordinary vegetative stages and until the 

 period of maturity, when two cells place themselves side by side wnthin 

 a common cyst. The two nuclei of each cell then unite, forming a 

 single synkaryon in each cell. The two adjacent cells finally unite 

 by dissolution of the cell w^alls that separate them, and the recently 

 fertilized nuclei, after some very questionable so-called maturation 

 processes, assume the characteristic position of the vegetative forms. 

 Here, then, if this observation is accurate, there is an exogamic fertili- 

 zation, but the end stage does not occur until the next following period 

 of maturity (Fig. 64). 



In the majority of protozoa the germ of the new individual, as in 

 metazoa, is produced by the union of cells from different ancestors, 

 and these cells, for the most part, show characteristic evidences of the 

 period of maturity. In some cases there is but slight difference, if 

 any, between the conjugating cells and the normal ones, the conditions 



