Embryology. 127 



The distinctively modern era in the history of ferti- 

 lization dates from about 1875, when the brilliant re- 

 searches of Auerbach, E. van Beneden, Butschli, Fol, 

 O. Hertwig, and others, showed that one of the 

 essential phenomena in fertilization is the intimate and 

 orderly association of the sperm-nucleus, of paternal 

 origin, with the ovum-nucleus, of maternal origin, the 

 result being the cleavage or segmentation-nucleus. The 

 researches of Strasburger, De Bary, and others estab- 

 lished the same result in regard to plants. 



Subsequent research has been mainly concerned with 

 deciphering the details of each step in the fertilization 

 process, and with the attempt to ascribe a role or func- 

 tional meaning to the different parts of the intricate 

 cellular mechanism concerned in the act. 



Although maturation precedes fertilization in time, 

 its significance was longer in being appreciated. In 

 1824 C. G. Carus observed that little bodies 

 (polar bodies, directive corpuscles, &c.) were 

 given off by the ripe ovum of the water-snail Limnceus: 

 Fr. Miiller and Lov6n made the same observation in 

 1848, and similar results gradually accumulated. In 

 1875 Biitschli showed that these little bodies were formed 

 by the division of the ovum -nucleus, and Fol con- 

 firmed this a year afterwards. It was soon shown that 

 in the majority of ripe ova it was a normal occurrence 

 that the unfertilized nucleus should divide twice in rapid 

 succession. In 1876 Giard interpreted the little bodies 

 as abortive ova, a view which Mark also emphasized 

 somewhat later (1881); and various other suggestions 

 were made as to their meaning. In 1883, however, 

 Van Beneden made the suggestive discovery that the 

 sex-nuclei, which become intimately associated in the 

 fertilization of the egg of the round-worm of the horse 

 (Ascaris megalocephala), contain each one-half the number 

 of nuclear elements or chromosomes characteristic of 

 the body-cells of the species, and this has been confirmed 

 in regard to numerous animals and plants. This led 

 on to Weismann's theoretical interpretation, that the 

 formation of polar bodies, and the analogous processes 

 in the history of the spermatozoon, involved " reducing 



