Thorn — Fertilization in Aspidium and Adiantum. 299 



The whole nucleus now seems to be irregularl}' crescent- 

 shaped, with its concavity parallel with the concave depres- 

 sion in the outside of the cell (figs. 6, 7, 8, 10, 14, 25). 

 Careful tracing of the nuclear membrane shows it still 

 sharp, but thin and thrown into folds like pseudopodia. 

 The difference in staining reaction between the nuclear 

 material and cytoplasm makes it possible to follow the 

 delicate pseudopod-like branches of the nucleus through the 

 cytoplasm. Such a branching condition of the nucleus ren- 

 dered adequate representation of it in drawings unsatisfactory, 

 so finally the plan adopted was to fix upon some point in the 

 section, sometimes the upper cut surface, sometimes what I 

 determined was the optical section, again some point which 

 was important, and draw the outline of the nucleus in that 

 plane. Within such figures I have represented the sperma- 

 tozoids as they enter, as nearly as possible in perspective 

 maintaining their true relation to the plane of the figure. 

 Sometimes it was necessary to represent by a change of shade 

 more than one plane of the egg (figs. 13, 17, 25) because 

 a portion of the spermatozoid lay in the cytoplasm outside the 

 nucleus but was partially overlaid by it in the plane of the fig- 

 ure. At its maturity, then, the egg contains a large amount of 

 cytoplasm which appears as a granular network staining evenly 

 for the most part, yet showing in places larger granules or 

 microsomes whose nature has not been determined. The 

 cytoplasmic network often shows an irregularly radial 

 arrangement into fibers and rows of granules directed out- 

 ward from the nucleus. The shape of this mass of cytoplasm 

 is determined by the cavity in which it lies. Within it is 

 the irregularly branching nucleus which usually occupies the 

 center of the cell body. It approaches nearest to the surface, 

 however, at the base of the depression which is filled in the 

 earlier stages by the ventral canal-cell. This is the point 

 where fertilization occurs, for here the entrance of the sper- 

 matozoid nucleus into the egg nucleus is most easily accom- 

 plished. 



Fertilization. 



The actual fusion of the two nuclei has been observed in 

 only a few plants. In 1869 Strasburger observed the entrance 



