220 



FERTILIZATfOX OF THE OVUM 



thus effects fertilization ( l^'ijj,'. 107). TJic ctJicr conjugates zvit/i one of 

 the polar nuclei (usuall\- tlic upper), wliich then unites with the other 

 polar nucleus {cf. jx 264). l^y di\-ision of the fertilized egg arises the 

 enibrvo ; wliile h\- dixision of tlic 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 As in the case of animals (p. 

 176), the germ-nuclei of phanero- 

 gams also show marked differ- 

 ences in structure and staining-reac- 

 tiun before their union, though they 

 ultimately become exactly equiva- 

 lent. Thus, according to Rosea 

 ('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 diflerences 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, Za?nia. [WEBBER.] 



A. Spermatozoid. B. The same after entrance into 

 the egg, showing nucleus («) and cilia-bearing band (;:). 



C. The ovum shortly after entrance of the spermatozoid. 



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

 periphery (c). 



