OOSPORES. 
^73 
swarming about (Fig. 178, jB). It also sometimes occurs that the gonidia clothe them- 
selves with a delicate cell-wall while still within the mother-cell, forming a kind of 
parenchyma, and then escape from the mother-cell through numerous holes in its wall. 
These various modes of formation of the gonidia, by means of which species and even 
genera have hitherto been distinguished, can, as Pringsheim has shown, take place 
simultaneously on the same plant, as in Saprolegnta and Achlya. In Saprolegn'm^ 
when the gonidia have escaped from the terminal cell of a branch, the septum 
bulges out and developes into a new gonidial receptacle which takes the place of the 
one that is now empty; in Achlya a lateral branch beneath the septum becomes the 
new gonidial receptacle. In the very small and simple genus Pythium, which is parasitic, 
filaments emerge which open at their apex and allow the protoplasm to escape in a ball 
which then breaks up into a number of zoogonidia. After coming to rest the zoo- 
gonidia of the Saprolegnieae give rise to new plants, which, when they have access to 
a fresh substratum, such as a dead fly, produce in succession several generations of 
non-sexual gonidia-forming individuals. 
It is only towards the close of the period of growth that sexual individuals make 
their appearance. The extremities of the filaments then swell up into a globular 
form (Fig. 179), a septum is formed beneath the swelling, which constitutes the oogo- 
nium, and its protoplasmic contents either simply contract to form the oosphere, as 
in Pythium monospermiim and Aphanomyces, or the contents divide into two or more 
T 
