ORGANIC EVOLUTION 



153 



into a very long pollen tube (fig. 97). This grows down 

 the style and enters the niicropyle of the ovule and liber- 

 ates the two sperm nuclei (that have escaped from their 

 own cell wall) near the egg nucleus. 



Fig. 98. Diagrams of development of the female game- 

 tophyte in the violet, p, a young ovule: / and 2, 

 inner and outer integuments; m, micropyle; n, 

 macrosporangium (nucellus) ; s, macrospore; q, the 

 female gametophyte at the time of fertilization; p, 

 pollen tube entering; e, egg nucleus; x, two 

 synergid nuclei; y, two endosperm nuclei; s, three 

 antipodal nuclei. Other lettering as in p. 



The macrospore, which is situated (as in the pine) within 

 the nucellus, here- surrounded by two investing integu- 

 ments, develops by three successive divisions of its 

 nucleus into an aggregation of eight nuclei within a distend- 

 ed cell wall, the w^hole constituting the so-called embryo 

 sac. The position and names of these are indicated 

 in the accompanying diagram (fig. 989). One of the 

 eight is the egg nucleus, and with it one of the sperm 

 nuclei from the entering pollen tube fuses. There is 

 no archegonial wall. The remaining cells represent the 

 body of the female prothallium, so far as developed at 

 time of fertilization. Among these occur two puzzling 

 phenomena, which render identity of parts somewhat un- 



