i8o 



FERTILIZATION OF THE OVUM 



crossing produces both greater vigour and greater variability. In view 

 of all the facts, however, we are constrained to the admission that the 

 essential nature of sexual reproduction must remain undetermined until 

 the subject shall have been far more thoroughly investigated, espe- 

 cially in the unicellular forms, where the key to the ultimate problem 

 is undoubtedly to be sought. 



A. PRELIMINARY GENERAL SKETCH 



Among the unicellular plants and animals, fertilization is effected 

 by means of conjugation, a process in which two individuals either 

 fuse together permanently or unite temporarily and effect an exchange 



Fig. 89. Fertilization of the egg of the snail, Physa. [KOSTANECKI and WIERZEJSKI.] 

 A. The entire spermatozoon lies in the egg, its nucleus at the right, flagellum at the left, while 

 the minute sperm-am phiaster occupies the position of the middle-piece. The first polar body has 

 been formed, the second is forming. B. The enlarged sperm-nucleus and sperm-amphiaster lie 

 near the centre; second polar body forming and the first dividing. The egg-centrosomes and 

 asters afterward disappear, their place being taken by those of the spermatozoon. 



of nuclear matter, after which they separate. /;/ all the higher forms 

 fertilization consists in the permanent fusion of tzvo germ-cells, one of 

 paternal and one of maternal origin. We may first consider the fer- 

 tilization 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. 



