Life H i story & Cytology of Synchytrium endobioticum. 91 
pass into the upper half of the host cell. The nucleus divides 
repeatedly and the extruded mass segments into five to seven 
portions; each segment is the initial of a zoosporangium and the 
whole is a young sorus. Nuclear divisions continue and finally 
the so-called zoospores are formed; when these are nearly mature 
the sporangia absorb water and swell, the soral envelope and host 
cell walls are ruptured, and the sporangia are set free on the 
irregular surface of the tumour. 
The motile cells, hitherto described as zoospores, are now 
liberated. They are smaller than the zoospores of the resting 
sporangium but similar in shape. These motile cells are facultative 
gametes for when the contents of different sporangia are discharg- 
together, which frequently occurs, fusion takes place and a zygote 
is formed. The gametes however may infect the host without 
fusion. The result of infection by the zygote, which becomes 
uninucleate before entry, is a resting sporangium ; the result of 
infection when fusion is omitted is a sorus. 
A tumour may bear several series of sori before the production 
of resting sporangia becomes general. As, however, the development 
of the sorus may be completed and its gametes liberated in less 
than three weeks while the growth of the resting sporangium 
extends over months, and, as in addition, the tumours when bearing 
only sori are small and liable to be overlooked, it is not surprising 
that the occurrence of sori as a regular stage in the cycle of 
development of the fungus has not hitherto been recognised. 
Imperial College of Science and Technology, 
South Kensington, London. 
