926 MURIEL ROBERTSON. 
from the same sphere conjugate together, but I have never 
seen this occur in all the spores from any one sphere; some 
individuals were always seen to swim away singly. In 
colonies where the spheres of several individuals had seg- 
mented, I observed conjugation of gametes at some distance 
from the place of segmentation of the spheres, but had no 
means of making sure that the conjugating individuals had 
really arisen from different Pseudospora. Conjugation on 
one occasion certainly took place between individuals from 
different spheres which had arisen in the same Pseudospora. 
In this case the conjugating gametes were unequal in size, 
but that this was of no special significance was shown by 
later observations, in which the gametes were equal. On 
account of the small size of the nuclei and the difficulty of 
observation, I am unable to say anything about the fusion of 
the gamete nuclei. 
In the many sphered individuals the essential processes of 
the development of the spheres correspond with those already 
described in the case of the single-sphered Pseudospora. 
Two or three spheres may arise inside the same nuclear space 
(fig. 21); this possibly is to be regarded as a precocious 
segmentation of the single sphere, though I am unable to say 
what is the cause. In other cases the nucleus appears to 
divide before the formation of spheres, and from each of 
these nuclei is formed one or more spheres. Occasionally in 
these individuals only one of the nuclei becomes converted 
into asphere, the other apparently disintegrating. 
The cultures in which the process of sphere formation was 
observed were kept at an almost even temperature of 
11°-18° C. and well supplied with food material. In two 
cultures of uninfected Volvox which were inoculated with 
sphere-forming individuals it was found that Pseudospora 
reproduced by fission for exactly fourteen days and then 
again formed spheres. In one culture this period repeated 
itself thrice in succession. In other cases the period varied 
from fourteen days to about twenty-one. ‘The formation of 
gametes is often nearly synchronous throughout a culture, 
