120 Bristol . — Life-history and Cytology of Chlorochytrium grande. 
to the simultaneous division of the contents of the zoogonidangium into the 
required number of parts. 
These differences seem to justify the separation of the alga as a new 
species of Centrosphaera to be named C. grande \ if the genus Centrosphaera , 
Borzi, can be regarded as an independent genus. 
As was mentioned at the beginning of this paper, the identification of 
algae belonging to this group is exceedingly difficult because of the great 
similarity of the genera. A careful examination of the original papers and 
figures by Klebs 1 and Borzi 1 does not reveal a single fundamental character 
by means of which the genera Chlorochytrium , Cohn, Endosphaera , Klebs, 
Scotinosphaera , Klebs, and Centrosphaera , Borzi, may be distinguished from 
one another in either the vegetative or the zoogonidangial states. In their 
habit all are more or less endophytic in water plants ; the first three genera 
live within the tissues of such plants as Potamogeton , Hypnutn , and 
Sphagnum , while Centrosphaera is frequently found growing among the 
filaments of colonies of blue-green algae, though it may be free floating on 
the substratum. Since the habit, the vegetative cells and the zoogonidangia 
are almost identical in all the genera, generic distinctions have been based 
by Klebs and Borzi upon details in the life-history. Under present circum- 
stances it is therefore necessary to follow the complete life-history of one of 
these algae, a task which may require any time from several months up to 
two years for its completion, in order to decide with any degree of certainty 
to which genus it should be assigned. Even then, the generic distinctions 
are in some cases so trivial that they might very easily be overlooked. 
For example, Chlorochytrium , Cohn (1874), and Endosphaera , Klebs 
(1881), both propagate themselves by means of spherical zoogonidia produced 
as the result of successive bipartition of the contents of the zoogonidangium. 
In both, the zoogonidia produced from a single cell fuse in pairs near the 
opening of the zoogonidangium to form zygotes. The distinction between 
the two genera is based upon the fact that in Chlorochytrium the zoogonidia 
are produced as the final products of an uninterrupted bipartition, while 
in Endosphaera the products of the first five or six divisions provide them- 
selves with a thin wall before they continue to divide, and about eight 
zoogonidia are produced from each of these secondary cells within the 
zoogonidangium. But no resting period appears to occur during this 
interruption in division. The only other distinction between the genera is 
a detail in the development of the zygote. In both genera the zygote rests 
upon the surface of a leaf and puts out a tube which penetrates between the 
cells of the epidermis, and down which the contents of the zygote pass to 
form at the end of the tube the endophytic cell typical of the alga. In 
Chlorochytrium the part of the zygote which lies on the surface of the leaf 
1 loc. cit. 
