OF MICRO-ORGANISMS. 83 



us. Finally, at the twentieth minute, the two nuclei 

 have united. 



In the psychical history of animal fecundation as 

 just given, there are many gaps: the history of vege- 

 table fecundation will fill several of these. 



The simplest forms of sexual reproduction in veg- 

 etables are those where the male and female cellules 

 are quite the same and advance to meet each other 

 equally; thus possessing not only the same form, but 

 also the same properties. In a small Alga bearing the 

 name of Ulothrix serrata, the interior of certain cellules 

 divides into two parts, which separate, then come to- 

 gether again and mingle anew into a minute mass 

 which, when set at liberty, reproduces the plant entire. 

 In other species the inside divides into small naked 

 cellules, which are first set at liberty and for some 

 time move briskly about in the water by means of cilia 

 with which they are provided, before fusing into one 

 another. These cellules are called zoospores. The 

 differentiation is further marked in certain species, the 

 zoospores of which have neither the same form nor the 

 same properties. Some leave their positions to go to 

 meet the others: these are the male cellules, the an- 

 therozoids; others make no movement at all and limit 

 their role to that of waiting: these are the zoospores. 

 Similarly, in an Alga bearing the name of Sphceroplea 

 annultna, there are found two kinds of filaments, 

 brown and green. In the green filaments the proto- 

 plasm of certain cellules breaks up into a definite num- 

 ber of ovoid bodies which remain immobile; in the in- 

 terim, the cellules of the brown filaments liberate mo- 

 bile spores provided with two flagella: these spores, 

 veritable male cellules, ply briskly about in the water 

 and then proceed to fix themselves to the green fila- 



