FERTILIZATION 



293 



Riccia, Meyer (1911) on Comma, Graham (1918) on Preissia, and 

 Woodburn (1920) on Reboulia. It appears that in bryophytes the body 

 of the biciliate spermatozoid, which consists mainly of nuclear material, 

 undergoes in the egg cytoplasm a transformation into a reticulate nucleus 

 before fusing with the egg nucleus (Fig. 117). The fate of the non- 

 nuclear structures (cytoplasm, blepharoplast, and cilia) is not known 

 with certainty, but it is probable that they are absorbed in the egg cyto- 

 plasm. In the liverwort, Preissia 

 quadrata, Miss Graham has found 

 two centrosomes with weakly de- 

 veloped asters in the cytoplasm of 

 the egg at the time the two pronuclei 

 are about to fuse (Fig. 23, A). It is 

 not known what relation their ap- 

 pearance may have to the entrance 

 of the spermatozoid. 



The most detailed account of fer- 

 tilization in a pteridophyte is that 

 given by Yamanouchi (1908) for 



M 



FIG. 117. FIG. 118. 



FIG. 117. Fertilization in Anthoceros. Male and female pronuclei about to fuse in 

 lower part of egg in venter of archegonium; elongated plastid above them. Gametophyte 

 cells show one nucleus and one plastid each. X 1050. 



FIG. 118. Fertilization in Nephrodium. 



A, spermatozoid entering egg nucleus. B, spermatozoid becoming reticulate in midst 

 of female reticulum. (After Yamanouchi, 1908.) 



Nephrodium (Fig. 118). In Nephrodium the multiciliate spermatozoid 

 enters bodily into the egg nucleus with no previous alteration into the 

 reticulate state. Here it gradually becomes reticulate and irregular in 

 shape, until finally its limits are indistinguishable, the chromatic material 

 contributed by the two gametes apparently forming a single fine-meshed 

 network. 



Gymnosperms. Among living gymnosperms the Cycadales and 

 Ginkgoales are characterized by the possession of motile spermatozoids. 



