SEXUAL REPRODUCTION 83 



(b) that the mycelium of a homothallic species is 



bisexual ; 



(c) while the mycelium of a heterothallic species is 



unisexual ; 



(d) and further, that in the + and series of the 



heterothallic group are represented the two sexes. 



In Dipodascus albidus^ belonging to the Hemiasci, we 

 have an apparently primitive mode of sexuality, admitted 

 as such on account of the fusion of two nuclei to form 

 a fusion-nucleus, which by repeated division produces- the 

 nuclei round which the spores are formed. From a 

 prostrate septate, vegetative hypha, two blunt lateral out- 

 growths or gametes appear, separated from each other by 

 a septum of the mother-hypha. These lateral branches or 

 sexual cells are both multinucleate, and soon fuse together 

 at the apex. According to Juel the nuclei in the sexual 

 cells are indistinguishable from the vegetative nuclei, but 

 soon after the fusion of the cells a larger nucleus appears, 

 supposed to result from the fusion of two of the smaller 

 nuclei. This large fusion-nucleus gives origin to a large 

 but indefinite number of nuclei, larger than the vegetative 

 nuclei. These large nuclei become centres of spore-forma- 

 tion. The apex of the fused gametes increases in length, 

 forming an ascus-like body containing the spores. The 

 vegetative nuclei remain with the cytoplasm at the base of 

 the spore-sac. The number of spores contained in the 

 spore-sac or ascus is variable in different individuals. 



Fertilisation in Sphaerotheca Castagnei^ one of the 

 Erysipheae, belonging to the Ascomycetes, has been 

 studied by Harper. The antheridium and oogonium 

 originate as lateral branches from adjoining hyphae. Each 

 at first contains a single nucleus and dense protoplasm, 



