The discrepancy of these observations makes it apparent that we ought in the present state of our knowledge to be very cautious in making any general conclusion regarding the nature of the spermatic animalcules. It appears to be fully proved that some such animalculse always exist in the seminal fluid of animals when they are fit for propagation ; but it is by no means certain that they belong exclusively to the fluids which are the product of secretion in the tes ticle, for animalcules very similar to them in general appearance and in motions are to be found in various other fluids and organs of animals. In all those parts of the body in which mucous secretions are accumulated, ani malculas are formed, and in some of the lower animals the Cercarias of intestinal mucus are hardly to be distinguished from the animal culae of their seminal fluid.
Nor is it well ascertained that these animal cules belong exclusively to the fluid of the tes ticle, and do not sometimes occur in the secre tions of other parts of the generative organs. They exist no doubt frequently in the seminal fluid of the testicle, but some recent observa tions seem to shew that they are frequently imperfect in the fluid of that organ, and that in some animals at least they are not fully formed and do not acquire their powers of active mo tion till some time after the seminal fluid is secreted, and when it has passed from the tes ticle into other parts of the generative organs. On this account some hold, and with good reason, that they are to be regarded as the pro duct of reciprocal changes of the ingredients of the seminal fluid on one another, rather than as secreted along with that fluid directly from the bloodvessels of the testicle, as others have sup posed.
In conclusion, we would remark that in regard to the seminal animalcules having both the body and tail, such as those that may be seen in the dog, cat, rabbit, or other quadru peds, and which were described by the dis coverers and early observers of the seminal animalcules, no one who has had an oppor tunity of observing them carefully with a good lens magnifying three or four hundred diame ters can doubt for a moment that they bear a close resemblance to some of the Infusoria, and that both from their structure and motions they are with as much justice as the Infusoria to be regarded as distinct animal beings. With re gard to the other kinds above mentioned, or the changes they may undergo in different stages of their existence, farther investigations appear necessary to enable us to form an opinion.
Although the spermatic animalcules, like other Entuzoa, are formed only in living ani mals and may be regarded as dependent for life on those animals in which they occur, yet they retain their life for a time after they leave the body. Thus the spermatic animalcules of the Polecat, which Prevost and Dumas ob served with much attention, continued to move for fifteen or twenty minutes on the object stand of the microscope ; and these experimen ters state that when the seminal fluid is allowed to remain in the genital organs, the animalcules continue to live for fifteen or eighteen hours after the death of the animal. Their motions cease instantly when a strong electric spark is passed through the fluid containing them.
Immediately after the discovery of the semi nal animalcules, they were made the subject of very fanciful hypotheses, and were conceived to throw quite a new light upon some of the ob scure parts of the generative process. To the supporters of the theory of pre-existing germs, their discovery opened up the prospect of being able to trace backwards one link more than had previously been done in the chain of life which connects the parent and offspring. By some they were considered as the cause of sexual en joyment or venereal propensities. By others, the animalcules were held to be of different sexes, and,according as one or other gained the egg during fecundation, to give rise to a male or female offspring, and thus to determine its sex. They have been supposed by others to form the first rudiments of the foetus or lay the foundation in the germ of the egg from which the offspring is afterwards developed, and fecundation has thus been resolved into the simple passage of a seminal animalcule into the germinal part of the egg; and finally, one or two of the most fanciful of such dream ing physiologists have (as we had occasion to remark at a former part of our article) not failed to perceive on a sufficiently minute inspection of the animalcule, that it already possessed all the organs belonging to the mature con dition of the animal in the seminal fluid of which it existed ; compressed no doubt into a very small space, but from which it was easy to suppose the offspring to be formed by evo lution.• Such notions require no refutation. Let us rather pass now to the inquiry of how far ob servation and experiment have tended to throw light upon the essential circumstances upon which the fecundating property of the seminal fluid depends.