22O 



FERTILIZATION OF THE OVUM 



thus effects fertilization (Fig. 107). The other conjugates with one of 

 the polarnuclei (usually the upper), which then unites with the other 

 polar nucleus (cf, p. 264). By division of the fertilized egg arises the 

 embryo ; while by division of the compound nucleus resulting from the 



fusion of the polar nuclei 

 and the second sperm nu- 

 cleus are formed the endo- 

 sperm-cells, which serve 

 for the nourishment of the 

 embryo. This remarkable 

 double copulation within 

 the embryo-sac is without 

 a parallel and is of wholly 

 problematical meaning, but 

 in no way contradicts the 

 general rule regarding the 

 union of two germ-nuclei 

 to produce the embryo. 1 



1 As in the case of animals (p, 

 176), the germ-nuclei of phanero- 

 gams also show marked differ- 

 ences in structure and staining-reac- 

 tion before their union, though they 

 ultimately become exactly equiva- 

 lent. Thus, according to Rosen 

 ('92, p. 443), on treatment by 

 fuchsin-methyl-blue the male germ- 

 nucleus is " cyanophilous," the 

 female " erythrophilous," as de- 

 scribed by Auerbach in animals. 

 Strasburger, while confirming this- 

 observation in some cases, finds the 

 reaction to be inconstant, though 

 the germ-nuclei usually show marked 

 differences in their staining-capac- 

 ity. These are ascribed by Stras- 

 burger ('92, '94) to differences in 

 the conditions of nutrition ; by 

 Zacharias and Schwarz to corre- 

 sponding differences in chemical 

 composition, the male nucleus being 

 in general richer in nuclein, and the 

 female nucleus poorer. This dis- 

 tinction disappears during fertiliza- 

 tion, and Strasburger has observed, in the case of gymnosperms (after treatment with a 

 mixture of fuchsin-iodine-green), that the paternal nucleus, which is at first " cyanophil- 

 ous," becomes " erythrophilous," like the egg-nucleus before the pollen-tube has reached 

 the egg. Within the egg both stain exactly alike. These facts indicate, as Strasburger 

 insists, that the differences between the germ-nuclei of plants are, as in animals, of a 

 temporary and non-essential character. 



Fig. 108. Fertilization in a cycad, Zamia. [WEBBER.] 



A. Spermatozoid. B. The same after entrance into 



the egg, showing nucleus (n) and cilia-bearing band (c). 



C. The ovum shortly after entrance of the spermatozoid. 



D. Union of the germ-nuclei, cilia-bearing band near 

 periphery (c). 



