130 FERTILIZATION OF THE OVUM 



quately confirmed, have yielded very strong evidence that in these 

 unicellular animals, even under normal conditions, the processes of 

 growth and division sooner or later come to an end, undergoing a 

 process of natural " senescence," which can only be counteracted by 

 conjugation. That conjugation or fertilization actually has such a 

 dynamic effect is disputed by no one. What is not determined is 

 whether this is the primary motive for the process i.e. whether 

 the need of fertilization is a primary attribute of living matter or 

 whether it has been secondarily acquired in order to ensure a mixture 

 of germ-plasms derived from different sources. The latter view has 

 been urged with great force by Weismann, who rejects the rejuve- 

 nescence theory in toto and considers the essential end of fertilization 

 to be a mixture of germ-plasms (" Amphimixis ") as a means for the 

 production, or rather multiplication, of variations which form the 

 material on which selection operates. On the other hand, Hatschek 

 ('87, i) sees in fertilization exactly the converse function of cJiecking 

 variations and holding the species true to the specific type. The 

 present state of knowledge does not, I believe, allow of a decision 

 between these diverse views, and the admission must be made that 

 the essential nature of sexual reproduction must remain undetermined 

 until the subject shall have been far more thoroughly investigated, 

 especially in the unicellular forms, where the key to the ultimate 

 problem is undoubtedly to be sought. 



A. GENERAL SKETCH 



Among the unicellular plants and animals, fertilization is effected 

 by means of conjugation, a process in which two or more individuals 

 permanently fuse together, or in which two unite temporarily and 

 effect an exchange of nuclear matter, after which they separate. In 

 all the higJier forms fertilisation consists in the permanent fusion of 

 two germ-cells, one of paternal and one of maternal origin. We may 

 first consider the fertilization of the animal egg, which appears to take 

 place in essentially the same manner throughout the animal kingdom, 

 and to be closely paralleled by the corresponding process in plants. 



Leeuwenhoek, whose pupil Hamm discovered the spermatozoa 

 (1677), put forth the conjecture that the spermatozoon must pene- 

 trate into the egg ; but the process was not actually seen until nearly 

 two centuries later (1854), when Newport observed it in the case of 

 the frog's egg ; and it was described by Pringsheim a year later in one 

 of the lower plants, QZdigonium. The first adequate description of 

 the process was given by Hermann Fol, in I879, 1 though many 



1 See rflenogenie, pp. 124 ff., for a full historical account. 



