There is likewise another important question, respect ing the formation and subsequent detachment of the ve sicle, whether this process, although we may conceive that it usually depends upon the concourse of the sexes, is essentially connected with it, or whether it may not oc casionally take place from other causes. Upon the whole, it appears probable that this is the case, and that, at least, under peculiar circumstances, the vesicle is detached from the ovary without the co-operation of the male, and deposited in the uterus, where it remains, for some time, ready to be impregnated with the seminal secretion. Some physiologists have of late gone so far as to maintain that this is always the case ; that the evolution of the ve sicle is due to the powers of the female alone ; and that the sole effect of the male is to impregnate the ovum al ready lodged in the uterus. This, it must be acknow ledged, presents a much more simple view of the opera tion ; but we conceive that the observations and experi ments adduced in its favour scarcely warrant its adoption.
When the vesicle has received its impregnation from the male, the next operation is to attach itself to the in ternal surface of the uterus, by means of vessels produced for this purpose, which are afterwards prolonged into the umbilical cord, and consist of an arterial and a venous part, connected with the circulating system of the fwtus. To the uterine termination of the cord is attached the pla centa, a mass of loose cellular substance, containing a considerable quantity of blood, part of which appears to be more immediately connected with the mother, and part with the foetus ; the object of this structure is to produce the same change in the blood of the foetus, which, after birth, is effected by the lungs. There is, however, some doubt respecting the mode in which this is accomplished ; anatomists have not been able to detect any communica tion between the maternal and fixtal vessels in the way of direct anastomosis, and it has therefore been conjectured, either that the blood is conveyed front one set of vessels 4 P 2 to the other by a system of minute absorbents, or that, without any actual transmission of the fluid, the blood-ves sels attached to the mother are brought into contact with those of the foetus, so that their contents, being separated merely by thin membranes, are enabled to act upon each other, in the same way that the blood in the lungs is af fected by the air that is received into the pulmonary vesi cles.
It remains that we say a few words respecting the de gree in which the two sexes contribute to the production of the ketus. And on this point two very celebrated hy potheses, for a long time, divided the opinions of physi ologists, one ascribing the essential part to the male, the other, on the contrary, supposing the greater share to be long to the female. The first of these opinions originated with the microscopical discoveries of Leeuwenhoek and his friends, who found the male secretion to contain a number of animalcules; these they conceived to be the rudiments of the flews, and that the only office of the fe male is to afford them a suitable situation, where they might be deposited and brought to maturity. The exist
ence of these animalcules is generally admitted, although many of the accounts that have been published concern ing them must be received with some caution, and it is extremely doubtful what connexion they have with the continuance of the species. It has been stated, that simi lar kinds of bodies may be found in various other fluids ; even though it may be admitted that the substance in question is more disposed than many others to give rise to them.
The second hypothesis to which we have alluded, that of pre-existing germs, was the one that was adopted by Haller and many of his friends, and was at one time very generally embraced. It supposed that the ovary contains, ready formed, the rudiments of the fcetus, and that the only office of the male is to impart life to it, not by the addition of any new matter, but merely by operating as a stimulus, and exciting into action its dormant faculties. It follows, as a necessary consequence from this doctrine, that the original female of every species contained in her ovaries the germs of every one of her descendants, en veloped in each other, by a species of embuitement, as it has been termed ! The arguments in favour of this doc trine were derived from experiments, which proved the extreme minuteness of the quantity of male fluid which was sufficient to impregnate the female, and also from some anotomical considerations in regard to the manner in which the fcetus of oviparous animals is attached to the parts composing the contents of the egg. The main bulk of the egg is evidently the produce of the female, and as Some of these parts seem to be continuous to the chick, this, it is argued, must likewise be derived from the same source. But, in answer to these arguments, we may re ply, that the first is simply a consideration of quantity ; that we have no means of ascertaining a prior;, how much matter is necessary to compose the first rudiments of the fcetus ; and that, for any thing we know to the contrary, the smallest particle may be amply sufficient for this pur pose. And, with respect to the anatomical argument, we may observe that the parts in question, although they ap pear continuous, may not have been originally so, but that they are united by close apposition, so as to form one con tinuous body, as soon as they become visible to the eye, although, in the first instance, they were derived from dif ferent sources. We are then reduced to the most obvious and natural hypothesis, that which was defended by Har vey- under the title of Epigencsis, which supposes that the is produced by the union of a certain quantity of matter derived from both parents. In this way alone can we conceive why the offspring should be equally disposed to partake of the properties of both parents,—a fact estab lished by daily observation, and which it appears impossi ble to reconcile with the hypothesis of animalcules, or pre-existing germs.