2. Uroidea, thread-like, in Mollusca, Am phihia, and some birds.
3. Cephal-uroidea, consisting of a globular and a tail part, in Mammalia, Birds, and In sects.
The first of these kinds of Spermatozoa are like the Monades among lnfusoria, the second resemble the Vibriones, and the third, as has been already remarked, the Cercaria.
It is important to remark that, in so far as has as yet been ascertained, the form and size of the spermatic animalcules do not bear any intimate relation to the animal in which they exist, nor to the ova of the female. In respect of form, Messrs. Prevost and Dumas state that the head is usually of a round lenticular shape in quadrupeds, while in most birds it is of a long oval shape ; but in some birds the form is the same as in most quadrupeds. The semi nal animalcules present nearly the same ap pearance in man and in the dog. Various markings are represented in the cephalic por tion of the animalcule' of some quadrupeds by Messrs. Prevost and Dumas, but these, we are inclined to believe, are not constant, and are appearances which have arisen from acci dental circumstances.
In respect to size, there appears to be still a greater want of correspondence. The semi nal animalcule: are said not to be larger in the whale than in the mouse. They are very much larger in Insects, Mollusca, and others of the lower animals than in Man. In the snail they are fifty-four times longer than in the dog, and considerably larger in the mouse than in the horse The following table exhibits appioximatively the sizes of the spermatic animalculm of some of the more common animals in parts of a 1 ine :* Parts of a line.
helix pomatia 410 Lymnens stagnalis 300 Aquatic Salamander .... 200 Viper 050 Polecat Guinea-pig Mouse 040 Linnet .. ..
Sparrow lledffehog 030 Anguis fragilis Bull 028 horse 1 025 Ass Goat Ram 020 Cat Rabbit . ..
Common fowl ........ 016 Frog 013 Dog j oos Nan (according to Der Gleichen) Man (according to Buffon) 006 Gruithuisen states that he has observed the seminal animalcules to propagate by division of their bodies, or fissiparous generation. But we are far from attaching implicit faith to all that has been stated even as matter of observa tion regarding these bodies.
We ought not to omit in this place to state another and a different view which has recently been taken of the nature of the moving particles of the semen ; we mean that of G. Treviranus, who, founding chiefly upon observations made by himself in the lower animals, as Mollusca and Insects, adopted the opinion that these particles are not independent animals, but ana logous in their structure and properties to the fibrils and particles occurring in the pollen of plants. Their motion he seems to regard as of the same kind with that discovered by R. Brown to exist in infusions of these and other minute floating particles, and not as of an animal or spontaneous kind. lie deduces this conclu sion principally from the alleged observation that the motion of the so-called animalcules is not the same as that of ordinary Infusoria, but differs from it in this respect, that it is simply vihratory and constant, and not interrupted by any of those stops or pauses and changes from place to place which are held to indicate spon taneity in the motions of Infusoria.t
Some of the facts already stated by us shew the fallacy of the opinion of Treviranus. Baer, who regards the Spermatozoa as distinct living animals, holds that Treviranus has observed only an imperfect condition of the animalcule, and states in the work of Burdach / some addi tional observations of his own made in the snail, which promise, when pursued further, to remove some of the difficulties respecting the nature of these bodies. Baer states that he has observed the head and tail parts to become separated from one another, and both these parts, but especially the tail, to move about after separation. Baer has observed also that there are various stages of formation and change of the seminal animalcule, during which not only their form but also their motions undergo remarkable alterations, and lie supposes that Treviranus must have observed the spermatic animalcule of the snail and mussel in one of these stages only. Observations made* by the author of this article and by Dr. in the frog, seem to bear upon this point, and to be in some degree confirmatory of the view given by Baer. We have almost invariably found, in observing the seminal fluid of the frog in the spring or summer, that the animal cules contained in it are not of the kind de scribed by authors in this animal, viz. with both head and tail, but of the thread-like form only. These were collected in bundles in the thick part of the fluid, and generally moved with a continued vibration such as we have previously described. In the thin part of the fluid there were a few round-shaped or monad like infusoria. Occasionally it happened that when water was added to the thick part of the fluid, and the bundles of the thread-like bodies were artificially broken down, some of them moved progressively through the fluid by the undulatory riggling of one of the extremi ties; and during their motion we were surprised to see some of those, which, when at rest, ap peared to be destitute of any cephalic part, fre quently assume the appearance of a head. This phenomenon we remarked to be owing to the circumstance that the end by which the animal cule moved forward was bent backwards on the middle of the body, so as at one time to give exactly the tadpole-like appearance which is represented as a head in their plates by Messrs. Prevost and Dumas. There could be no doubt that this was the case, for in some, in which at one time the end was so closely joined to the body that it could not be seen, at another it loosened from it, and the thread-like animal cule still continued to progress in the fluid with its curve forwards, and the two ends (of unequal length) floating separate and loose. The author has observed nearly the same phe nomena in the spermatic fluid of the pigeon. Lastly, the observations of It. Wagner on the spermatic fluid of the Guinea-pig seem to prove more decidedly than any of the previously men tioned facts that the spermatic infusoria are subject to remarkable changes of form at dif ferent periods, and that they even go through a regular gradation of development.