More recently another German physician has advanced the theory that the sex may be controlled at will by observing the time of fecundation. He asserts that when fecundation occurs shortly after menstruation, the result will be a female; but if impregnation occurs later in the month, and prior to the three or four days preceding the next menstrual period, a male will almost certainly be produced. This theory was proposed by Prof. Thury of the academy of Geneva, who claims to have thoroughly tested it in a great variety of ways, and always with an affirmative result. Dr. Heitzman, of New York, an instructor in pathological histology, and an eminent physiologist, informs us that he has thoroughly tested this theory, and finds it to be entirely reliable. There are numerous facts which seem to corroborate the truth of this theory, and future investigations may give to it the dignity of an established physiological fact. Heredity.--The phenomena of heredity are among the most interesting of biological studies. It is a matter of common observation that a child looks like its parents. It even happens that a child resembles an uncle or a grandparent more nearly than either parent. The same peculiarities are often seen in animals. The cause of this resemblance of offspring to parents and ancestors has been made a subject of careful study by scientific men. We shall present the most recent theory adopted, which, although it be but a theory, presents such an array of facts in its support, and explains the phenomena in question so admirably, that it must be regarded as something more than a plausible hypothesis. It is the conception of one of the most distinguished scientists of the age. The theory is known as the doctrine of _pangenesis_, and is essentially as follows:-- It is a fact well known to physiologists that every part of the living body is made up of cellular elements which have the power to reproduce themselves in the individual, thus repairing the damage resulting from waste and injury. Each cell produces cells like itself. It is further known that there are found in the body numerous central points of growth. In every group of cells is found a central cell from which the others originated, and which determines the form of their growth. Every minute structure possesses such a center. A simple proof of this fact is found in the experiment in which the spur of a cock was grafted upon the ear of an ox. It lived in this novel situation eight years, attaining the length of nine inches, and nearly a pound in weight. A tooth has been made to grow upon the comb of a cock in a similar manner. The tail of a pig survived the operation of transplanting from its proper position to the back of the animal, and retained its sensibility. Numerous other similar illustrations might be given.