138 CONJUGATION, MATURATION, AND FERTILIZATION 



in explanation. The phenomena of conjugation and maturation of 

 the germ plasm which accompany fertilization are more easily inter- 

 preted, for they are largely matters of observation and deduction. In 

 protozoa we have a particularly rich field for investigation of these 

 problems, for the union of germ plasms is accompanied by phenomena 

 of such relative simplicity that they are more easily observed, con- 

 trolled, and interpreted than with metazoa. 



In interpreting the phenomena of fertilization of protozoa we are 

 in accord with those naturalists who, since the time of Harvey, have 

 advocated some "dynamic" theory or other. (See Wilson, The Cell, 

 p. 178.) In recent times this explanation is usually based upon the 

 facts of decreasing vitality with advancing age, and, as expressed by 

 Hertwig, fertilization is the means of restoring to a labile condition 

 the protoplasm which, with continued physiological activity, has 

 become stable in physical and chemical equilibrium. It is, therefore, 

 essentially a process of rejuvenation. 



Opposed to this point of view are those who, with Weismann and 

 his followers, maintain that protozoa do not die of old age, and that 

 conjugation with fertilization is an incidental occurrence in the life 

 of a race. Fertilization, in higher forms, is a means of bringing about 

 variation within the species, and at the same time a means of keeping 

 the species true to its structural type. 



Weismann still maintained his contention in regard to the immor- 

 tality of infusoria after Maupas' classical experiments had demon- 

 strated old age, and held that conjugation does not alter the indi- 

 viduality of the cells, since that individuality is retained after con- 

 jugation. Such a point of view would seem to be, however, merely 

 an expedient to save the argument, for the essential part of the fer- 

 tilized protozoon, like the metazoon, results from the union of two 

 germ plasms, the protoplasm resulting from this union being a new 

 individual in both cases. Like the metazoon, the protozoon is physi- 

 cally immortal only in the same sense of continuity of the germ plasm, 

 for, with each fertilization there is a re-organization of the protoplasm, 

 new chemical and physical combinations, and new individuality. 

 There is no difference in kind in protozoa and metazoa, only a differ- 

 ence in degree. 



The essential feature of fertilization appears to be the union of two 

 masses of chromatin. We can only conjecture as to the significance 

 of such union, but whatever hypotheses are framed to explain it, they 

 must take into consideration a great variety of conditions under which 

 the phenomenon is manifested. It is quite evident that complicated 

 processes in metazoa are the highest and last steps, so to speak, in the 

 elaboration of this universal biological phenomenon, and it is probable 

 that they differ only in degree from the lowest and most primitive 

 steps shown by the simple syngamic processes in protozoa. 



