Such curious cases as the Carolina twins and Chang and Eng were formerly supposed to be the result of the union of two separate individuals. It is now believed that they are developed from a single ovum. It has been observed that the primitive trace--described in a previous section--sometimes undergoes partial division longitudinally. If it splits a little at the anterior end, the individual will have a single body with two heads. If a partial division occurs at each end, the resulting being will possess two heads and two pairs of legs joined to a single body. More complete division produces a single trunk with two heads, two pairs of arms, and two pairs of legs, as in the case of the Carolina twins. Still more complete division may result in the formation of two perfect individuals almost entirely independent of each other, physiologically, but united by a narrow band, as in the remarkable Siamese twins, Chang and Eng. In a curious case reported not a great while ago, a partially developed infant was amputated from the cheek of a child some time after birth. The precise cause of these strange modifications of development is as yet, in great degree, a mystery. Hybrids.--It is a well-known law of biology that no progeny result from union of animals of different species. Different varieties of the same species may in some cases form a fertile union, the result of which is a cross between its two parents, possessing some of the qualities of each. The mule is the product of such a union between the horse and the ass. A curious fact is that the offspring of such unions are themselves sterile almost without exception. The reason of this is that they do not produce mature elements of generation. In the mule, the zoosperms are either entirely absent or else very imperfectly developed; hence the fact that a colt having a mule for its sire is one of the rarest of curiosities, though a few instances have been reported. This is a wise law of nature to preserve the purity of species. Law of Sex.--If there is a law by which the sex of the developing embryon is determined, it probably has not yet been discovered. The influence of the will, the predominant vitality of one or the other of the parents, and the period at which conception occurs, have all been supposed to be the determining cause. A German physician some time since advanced the theory that the two testicles and ovaries produce elements of different sexual character, the right testicle forming zoosperms capable of producing only males, and the right ovary producing ova with the same peculiarity. The left testis and the left ovary he supposed to form the female elements. He claimed to have proved his theory by experiments upon animals. Even if true, this theory will not be made of practical importance. It is, in fact, nothing more than a revival of an old theory held by physicians who flourished more than two thousand years ago.