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REVIEWS AND BOOK NOTICES. 687 
elements, we ought to know their life-history; and if this be 
desirable in the question of abiogenesis, it must be absolutely 
essential before we can even approach that of heterogenesis. We 
must patiently follow them without a break in observation, through 
all their changes, and then, by repeating these observations, 
decide on the stability or otherwise of the form. For some years 
our attention has been individually directed to this subject; and 
three years since the advisability of combined work commended 
itself to us. Kor work of this kind to be effective, we believe 
there must be more than one observer, in order that the observa- 
tions may be unbroken as far as possible, and also to secure a 
mutual as well as a double confirmation.” 
With Ross’ medium powers, and Powell and Lealand’s high 
ones, the authors commenced: the study of an undescribed monad 
which sometimes occurs abundantly in water in which a cod’s 
head has been macerated. The drop of infusion was so arranged 
that it could be preserved in the focus of the highest powers, 
and the organisms inhabiting it maintained alive and healthy, and 
under continuous observation for an indefinite length of time. 
The cercomonad subjected to study was a small oval body with 
two actively moving flagella at one end. This was the familiar, 
mature form, and the one which, alone, according to the usage 
of the students of Infusoria, would be considered characteristic of 
the species. Other forms however were observed, differing in size 
and shape and with one flagellum at each end, or amceboid with or 
without flagella, or cyst-like and smooth and globular; forms each 
of which might easily be regarded as a distinct species or possibly 
as a capricious variety, but which were tracked through a series 
of transitions, the recurrence of which was repeatedly observed 
and was found to be unvarying and to be a portion of the life 
history of the same individual. The mature form with oval body 
and two flagella at one end, after moving about with great activity 
for a period of time which in the observed cases was about forty 
Minutes, became squarer or more elongated, and somewhat dumb- 
bell shaped by a sudden constriction of the sarcode. At this 
Stage the body is furnished with one flagellum at each end, which 
lashes with great force. The constricted portion becomes nar- 
rowed more and more by stretching until so attenuated as to equal 
Only the flagella in thickness, when it parts in the middle, leaving 
two separate bodies each furnished with a flagellum at each end. 
