252 C. O. WHITMAN. 



this moment the tail of the spermatozoon is seen projectincr 

 from the yolk, and the point of penetration has become a clear 

 spot {" taclie clair ") which soon forms the centre of a radial 

 system. This body is the male pronucleus. As soon as the 

 spermatozoon has entered the yolk, the clear disc, from the 

 centre of which the cone arose, begins to extend in all directions 

 from the point of penetration, and ends by enveloping the entire 

 yolk. Tliis envelope is the vitelline membrane. In normal 

 cases this membrane forms very rapidly, thus preventing the 

 admission of more than one spermatozoon into the yolk. In 

 abnormal cases, when it forms more tardily, several spermatozoa 

 reach the yolk, and the membrane forms from as many centres. 

 In such cases the several male pronuclei unite one after the 

 other with the female pronucleus, but the cleavage is always 

 abnormal .1 



Strasburger (155) has studied the same phenomena in the 

 vegetable kingdom with no less success than Fol in the animal 

 kingdom. From these interesting researches of Strasburger it 

 is evident that the male pronucleus in Phanerogams (Picia and 

 Orchis) forms at the expense of the contents ot the pollensack, 

 just as in animals it arises at the expense of the spermatozoon. 



Hoffmann \-^ was the first to report pronuclei for Clepsine, 

 but he was able to give no account of the origin of either of 

 these bodies, nor of their destination, and failed to find the 

 pronucleoli. 



The investigations thus far made, justify, I believe, the fol- 

 lowing general conclusion : fecundaiiou, throughout the organic 

 kingdom, consists in the coalescence of corresponding j}arts of a 

 pair of sexnallij- differentiated cells, to form a unicellular 

 asexual individual. It is a re-union, not oj exhausted, but of 

 complementary energies. 



c. Polarity. Hatscluk (vVt^ Pedicellina) has called atten- 

 tion to the universality and early appearance of polar difl'eren- 

 tiation in the egg ; and Hay Lankester ('jV) 'isis pointed out the 

 importance of the same in the evolution of multicellular organisms. 

 Its universality is attested by the production of polar globules, 

 by discoidal cleavage (Aves, Jleptilia, Pisces, Cej)lialopo(la), and 

 by unequal cleavage (Ain])liibia, Petromyzon, MoUusca, Vermes), 

 Even in cases of superficial or peripheral cleavage (Insects) such 

 a diil'erentiation is evinced both by the shape of the egg and by 

 the formation of poh-cells. The manifestation of this jjolarity 

 in the egg always follows the transformation of the germinal 



' June 19, 1S7S. Sileiika (' Befruclituiig dos Eies vou Toxopucusfcs 

 variegatvx,^ Eiic:clmanii, 187^) and Calberla (" Dcr Be,fruciitunirsv()ijriiii«,' 

 brim Ei von rdromyzon," 'Zcitsclir. f. wiss. ZdoI.,' B. xxx, p. '137) liave 

 butli traced the male pronucleus to a sin^^'lc bpernialozoon. 



