GYMNOSPERMS. 



'59 



During the final stages in the development of the spermatozoid the 

 proximal end of the pollen tube, which is still capped by the exine of 

 the spore, grows downward into the prothallial cavity as in Zamia 

 (Fig. 65, A). This cavity in Cycas, according to Ikeno, is filled 

 with a watery fluid derived largely from the archegonia, and in which 

 the sperrnatozoids swim on escaping from the pollen tube. Webber 

 is of the opinion that in Zamia this fluid is derived largely from the 

 pollen tube. 



The sperrnatozoids in Cycas, on escaping from the pollen tube, swim 

 about rapidly, and in a short 

 time penetrate the egg. 

 That part of the egg at which 

 a spermatozoid enters is de- 

 pressed, giving the impres- 

 sion that it came against the 

 egg with some force. The 

 nucleus of the spermatozoid 

 now escapes from its cyto- 

 plasmic mantle and migrates 

 toward the nucleus of the 

 egg. The cytoplasm and 

 blepharoplast are left in the 

 upper part of the egg as in 

 Zamia (Fig. 66, A, B), 

 where they undergo disor- 

 ganization. It frequently 

 happens that several sperrn- 

 atozoids reach the egg, but, 

 as a rule, only one penetrates 

 into its interior, the others 

 remaining at the surface. 

 Whether more than one male nucleus ever fuses with the egg-nucleus 

 is not known. 



When male and female nuclei come in contact they are readily 

 distinguished from each other, the male being smaller, with a more 

 finely granular threadwork. Both are in the resting stage. The male 

 nucleus seems to press against the female, forming a depression in the 

 latter. In a short time the male nucleus is completely imbedded within 

 the egg-nucleus ; the membrane of the male nucleus disappears, and 

 the two nuclei fuse so completely that the fusion nucleus can scarcely 

 be distinguished from an unfecundated nucleus of the egg. 



FIG. 66. Fecundation of egg-cells in Zamia. (After 

 Webber.) 



A, egg-cell immediately after coming together of male and 



female nuclei ; the ciliferous band of fecundating sper- 

 matozoid lies in upper end of egg ; a second spermato- 

 zoid trying to gain entrance is shown at apex of egg. 



B, similar to A, but showing longitudinal section of ciliferous 



band in upper end of egg. 



