REPRODUCTION. 



619 



that a male organ of this kind is not uncommonly distinguished 

 as a po llinodiiim. 



In the Peronosporeae the sexual organs differ in size, the 

 female organ (oogonium) being the larger, and, as in the 

 Zygomycetes and in the Ancylisteae, they are developed in 

 close relation with each other. But there is this well-marked 

 advance in sexuality that, as a preliminary to fertilisation, 

 there is a process of cell-formation in both of the organs. In 

 the pollinodium the protoplasm undergoes differentiation into 

 a delicate hyaline peripheral layer, which de Bary has termed 

 the periplasm, and a granular central mass, which he has 

 termed the gonoplasm. In the oogonium, similarly, a layer of 

 periplasm is differentiated from the granular central mass 

 which is the oosphere. Just before fertilisation, the pollinodium 

 puts out a small tubular protuberance which penetrates the 

 wall of the oogonium and extends to the oosphere. The apex 

 of this tube then opens and the gonoplasm of the pollinodium 

 passes, as a gamete, through the tube and enters the oosphere, 

 the product of the fusion being an oospore. (Fig. 72.) 



Fig. 72 (after de Bary). Sexual reproductive organs and fertilisation of 

 Pythium. a antheridium. og oogonium. os oosphere. In the right-hand figure 

 fertilisation is in progress : in the left-hand figure it is completed and the oospore 

 is formed. 



The sexual organs of the Saprolegnieae closely resemble 

 those of the Peronosporeae. In the oogonium a process of 

 cell-formation takes place which results in the production of 

 one or more oospheres ; but there is this difference between 

 this process and that which takes place in the oogonium of the 



