of Bacteria and their Allies by Heterogenesis. 399 
fortions that have been saturated with the germicidal fluid 
in which they had been for a time soaked *. 
As J have previously pointed out, the existence of ‘ latent 
germs ”’ in the substance of healthy fruits and vegetables is 
not assumed—it is, in fact, expressly denied (pp. 383, 390). 
Hence the great weight to be attached to the preceding 
observations as evidence that the various microorganisms 
found within the cells have actually originated there by 
heterogenesis. 
It remains to be seen what evidence of similar cogency 
can be obtained in regard to the origin of microorganisms 
within the tissues of animal organisms. 
It would be useless to multiply instances. I will there- 
fore first cite a single case in which the origin of Bacteria 
may be actually watched within the body of a low animal 
organism, and then turn to their mode of appearance within 
some of the tissue elements of different vertebrates. 
Evidence of a particularly convincing nature is to be 
obtained from the examination of a little creature low in the 
scale of animal life, namely, Cyclops quadricornis, one of 
the Entomostraca so commonly to be tound in ponds. It 
may be seen from pl. xxiv. of Baird’s ‘ Natural History 
of the British Hntomostraca’t that the four pairs of 
abdominal feet and also the tail are furnished with a number 
of “ plumose spines or sete.” 
Examination of one of these organisms will show that within 
the chitinous envelope of these slender spines, which taper 
away to sharp points, there is nothing but structureless proto- 
plasm to be seen (Pl. XX VI. fig. 9, A, x 700). If we take one 
of these little creatures, put it in a drop of distilled water ona 
glass slip, with a tiny fragment of a No. 2 cover-glass on each 
side of it, and place over all a cover-glass, it will be found that 
the animal is soon killed by the weight of the latter, though the 
fragments of glass prevent rupture of the body. We may then 
place the microscope-slip in a Petri dish containing a thin 
stratum of water (so as to prevent evaporation from beneath 
the cover-glass), and fixing upon one of the tail-sete, which 
are larger than those on the abdominal feet, we may examine 
it from time to time. What may be observed is this :— 
After an interval of two or three days (the duration 
depending upon the temperature of the air at the time), we 
* In these cases the organisms often have to be long and carefully 
searched for, A perfunctory examination would almost certainly lead to 
the statement that no organisms were present. 
+ Ray Society, 1850. 
