DISEASES OF THE DIGESTIVE ORGANS. 219 
admissicn of this explanation be sufficient ; for many of the entozoa 
are not propagated by eggs, but belong to the viviparous class, 
so that in regard to them the difficulty remains undiminished. 
But granting the existence of ova without, and their reception into 
the body, it is still impossible to explain the development from 
them of the animals found in the parenchyma, in the embryo, etc., 
without, at the same time, admitting ‘that the ova are not only 
carried to these localities through the blood-vessels, but actually 
pass through the walls of the capillaries. Such an admission 
would be a physiological absurdity; for the extreme vessels will 
allow of the passage of a single blood-globule at a time, and no 
more, and will not permit any denser fluid than the plasma of the 
»lood to permeate their walls. How, then, could they afford a 
passage in any manner to ova, the least of which is ten times aa 
large as a blood globule? 
If the hypothesis now presented is untenable, it only remains 
to adopt the alternative one, to-wit: that entozoa are generated or 
created anew out of the materials or the products of the living 
organism. It may be urged affirmatively, in support of this doc- 
trine, that each organ possesses its own entozoa—the kidney, a 
species <lifferent from those of the intestine, which are, again, un- 
like the parasites of the liver. Even more: the several parts of 
the same organ generate dissimilar animals. The small intestine 
produces the round and the tape-worms; the large intestine, the 
two species of thread-worms. These facts seem to show that some 
extremely local concurrence of circumstances is essential to the 
production of the several entozoa. It may also be argued, and we 
think the argument unanswerable, that if spermatic animalcules, 
which exist in the testicle, are there spontaneously generated, no 
violence is done to probability in supposing parasitic animals to 
be produced in the same manner. It will hardly be denied that 
spermatozoa are literally evolved from the constituents of the 
aemen ; but it is objected to the doctrine of spontaneous generation 
that it is against analogy, which every-where supports the famous 
dogma, omne vivum ex ovo. This objection is a mere begging of 
the question. ‘The decision of the case in hand involves the truth 
of the theory just quoted, and, as we believe, must be allowed to 
show that this theory is not absolutély universal in its application. 
Other facts, also, among which are the following, tend to invali- 
date it. Nothing can he more certain than that all organized 
