Trow. — The Karyology of Saprolegnia . 647 
is a new generation intercalated between two successive 
gametophytes, and that it is entirely unrepresented amongst 
animals, it is not unlikely that the reducing division has in 
this case a special significance. 
In fact, I would suggest that the evolution of the sporo- 
phyte was initiated by the fusion of gametes in which the 
halving observed in animals and Thallophytes did not take 
place ; and that the increase in the mass of the nucleus had 
the effect of a stimulus in the direction of spore-formation, 
giving to the organism the advantages of polyembryony. 
The power of subsequent return to the undoubled condition 
must, on this theory, have been developed simultaneously 
with the sexual capacity of the abnormal gametes. Wallace 
has conclusively shown that such simultaneous variations 
certainly occur. 
Such a view is, at any rate, supported by the facts already 
ascertained, and will certainly have the effect of stimulating 
me, and possibly others, to further research. An investigation 
of the karyology of Oedogonium , Coleochaete , and some genera 
of the Florideae seems particularly desirable. 
The results of Klebahn’s investigation of Closterium and 
Cosmarium , and Chmielewsky’s investigation of Spirogyra , 
appear to require confirmation. The facts seem to me to 
support the theory propounded here. The whole course of 
development and the minutest details as to the structure of 
the nuclei must, however, be determined before the significance 
of the behaviour of the nuclei can be made clear. 
Summary. 
1 . The nucleus in the genus Saprolegnia is bounded by 
a nuclear wall, and possesses one central chromosome of 
spongy texture. The space between the nuclear wall and 
chromosome is occupied by a nucleo-hyaloplasm, which is 
traversed by fine threads. 
2. The nucleus undergoes direct division in the zoospore 
