The union of the two principles in the Womb appears so likely an event that it is scarcely possible to avoid thinking that it really does then take place, but at the same time there are very strong reasons for adopting the opinion that it takes place in the Ovary. Among others may be mentioned the phenomenon of _extra uterine conception_, which will be more fully alluded to in another place. In these cases the fœtus is found _outside_ of the Womb, in the Tube, or the Ovary, or even in the Abdomen, among the intestines. Such an occurrence seems to render it almost certain that the ovum must have been fecundated in the Ovary, because we cannot well conceive, if it were not so, how it could reach the outside of the Womb. This difficulty is not, however, regarded as insuperable, by the advocates of the opposite theory. Possibly conception may occur _in both ways_. There are many causes which prevent conception, or, in other words, which produce _barrenness_ and _sterility_. These various causes cannot be all explained, except in connection with a full explanation of the process of reproduction, and of the nature of many female diseases. In my work on "_The Diseases of Woman_," I have given a chapter on this subject, and have also referred to the present work as one in which it would be more fully treated. This reference, however, was made in mistake; the subject is not needed here, but will be in the other work, on the _reproductive functions_ in which it will be discussed at length. CHAPTER IV. FŒTAL DEVELOPMENT. The last Chapter brought us up to the point of conception, or the first commencement of the new being. The next step is to exhibit its various stages of development, and to show how it is nourished and maintained in its proper position. This is requisite in order to understand the origin of many diseases and accidents which occur during gestation, and also to explain the various signs by which it is determined whether a female is pregnant or not. FŒTAL DEVELOPMENT. The development of a perfectly formed human being from the egg in which it originates, is one of the most astonishing phenomena that can come under our observation, and is eminently deserving the study of every rational being. The present explanation of it must necessarily be brief, being merely intended to make the main subject more clear. It has already been remarked that it is uncertain whether the ovum is impregnated before it is brought into the Uterus, or after it arrives there; but be that as it may, nothing has yet been discovered in the Womb till several days after conception. Some physiologists tell us that the rudiment of the new being may be found there about the _sixth_ day; but others again assure us that it cannot be found before the _twelfth_, at which period our explanation of its development will commence.