PRELIMINARY GENERAL SKETCH l8l 



Leeuwenhoek, whose pupil Hamm discovered the spermatozoa 

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

 trate into the egg ; and the classical experiments of Spallanzani on 

 the frog's egg (1786) proved that the fertilizing element must be the 

 spermatozoa and not the liquid in which they swim. The penetration 

 of the ovum was, however, not actually seen until 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, CEdigonium. The 

 first adequate description of the process was given by Hermann Fol, 

 in I879, 1 though many earlier observers, from the time of Martin 

 Barry ('43) onward, had seen the spermatozoon inside the egg-enve- 

 lopes, or asserted its entrance into the egg. 



In many cases the entire spermatozoon enters the egg (mollusks, 

 insects, nematodes, some annelids, Petromyzon, axolotl, etc.), and in 

 such cases the long flageilum may sometimes be seen coiled within 

 the egg (Fig. 89). Only the nucleus and middle-piece, however, are 

 concerned in the actual fertilization ; and there are some cases 

 (echinoderms) in which the tail is left outside the egg. At or near 

 the time of fertilization, the egg successively segments off at the upper 

 pole two minute cells, known as the polar bodies ( Figs. 89, 90, Ii6)or 

 directive corpuscles, which degenerate and take no part in the subse- 

 quent development. This phenomenon takes place, as a rule, imme- 

 diately after entrance of the spermatozoon. It may, however, occur 

 before the spermatozoon enters, and it forms no part of 'the process 

 of fertilization proper. It is merely the final act in the process of 

 maturation^ by which the egg is prepared for fertilization, and we 

 may defer its consideration to the following chapter. 



I . The Germ-nuclei in Fertilization 



The modern era in the study of fertilization may be said to begin 

 with Oscar Hertwig s discovery, in 1875, of the fate of the sperma- 

 tozoon within the egg. Earlier observers had, it is true, paved the 

 way by showing that, at the time of fertilization, the egg contains 

 two nuclei that fuse together or become closely associated before 

 development begins. (Warneck, Biitschli, Auerbach, Van Beneden, 

 Strasburger.) Hertwig discovered, in the egg of the sea-urchin 

 ( Toxopneustcs lividius\ that one of these nuclei belongs to the egg, 

 while the other is derived from the spermatozoon. This result was 

 speedily confirmed in a number of other animals, and has since been 

 extended to every species that has been carefully investigated. The 

 researches of Strasburger, De Bary, Schmitz, Guignard, and others 

 have shown that the same is true of plants. In every known case an 



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



