SPERMATOZOA. 363 



gonium. Within its base it holds a globose cell, 

 which becomes the embryo-sac, while the elevated 

 portion becomes a sort of chimney or shaft, composed 

 of four courses of four cells each, the top of which is 

 as yet closed. The embryo-sac encloses a single 

 nucleus at first ; but presently a second is developed, 

 which becomes the germ-vesicle ; it is minute at first, 

 but rapidly increases, while the primary nucleus 

 shrivels and disappears. At length, the cells which 

 close the summit of the shaft burst apart, and its 

 fluid contents escape, leaving an open passage to the 

 embryo sac. At the same time, the wall of the latter 

 softens and dissolves, and the germ lies exposed at 

 the bottom of the open shaft. 



Let us now return to the spermatozoa, which are 

 whirling about in giddy gyrations in those minute 

 drops of condensed moisture, which lie studding the 

 surface of the prothallium, like seed-pearls. Often 

 these drops coalesce ; and the thin space between the 

 prothallium and the earth is a continuous stratum of 

 water. One of the spermatozoa, finding the summit of 

 an archegonium open, instantly enters, and makes its 

 way to the bottom, where the germ lies, around which 

 it plays sportively. This is impregnation : as soon as 

 it occurs, the end of these wonderful provisions is 

 attained ; and the shaft immediately closes again by 

 the swelling of the terminal cells. The germ further 

 enlarges, and divides into four cells in one plane, one 

 of which grows into the bud and first frond of the 

 young fern, while another produces the root. The 



