HEREDITY. So long as the part played i by the male and female in procreation was obscure, as it was to the ancients, it was im possible to arrive at a clear idea concerning the relationship of offspring to parents, but after Spallanzani's discovery that the solid part only of the sperm forms the fertilizing agent, and more especially after Hamm's identification of the spermatozoon, it became possible to study the method of inheritance more directly. At the time when these advances were made the doctrine of praeformation was still generally held and the attempt to harmonize the new discovery of the spermatozoon split the prae formation school into two schools. According to one side, the ovists, the egg contains in miniature a germ in all essential respects, ex cept size, like the parent. The spermatozoon only starts the vital processes but contributes nothing material to the result. Heredity, there fore, sensu strictu, could only take place through the female. According to the other side, the spermists, the spermatozoon contained the praeformed germ of the individual, and the egg supplied only the pabulum for its expan sion and growth. Both ovists and spermists were logically forced to the further assumption that each germ contained within itself other germs and these still others, and so, if not ad infinitum, still each contained a sufficient number of enclosures to insure its perpetua tion of the species as long as ts predestined course was to last.
This deadlock was broken with the discov ery of the cell (1838-39) and the part it plays in fertilization. Plants and animals have been found to be made up of modifications of a single unit called the cell — a mass of protoplasm usually surrounded by a membrane and con taining a central sphere or nucleus. The egg is such a cell — one so simple as compared with its many modifications in the body that the egg is almost an idealized type of the cell. Minute study of the spermatozoon and its origin re vealed that it, too, starts as a cell as simple as the egg cell. Fertilization was found to con sist of the union of these two cells, one from one parent and one from the other, at least in all animals and plants in which the sexes are separate. By union of the two cells a single cell is produced which through division gives rise to the host of cells of which the new indi vidual is produced. Every cell of the new individual contains all the hereditary elements derived from the two parents. These heredi tary elements are now recognized as contained in the cell in certain threads called chromo somes that are the essential elements of the nucleus of the cell. The number of chromo
somes is definite for each species of plant or animal. The numerical constancy is maintained by a division that takes place in the ripen ing of the egg and of the sperm mother cell, for at this time a process occurs that need not be further described here (see below), which leads to a reduction in the number of chromo somes to a single set — one of each kind. By the union of these reduced cells in fertilization the double number of chromosomes is again restored. This demonstration of the duality of the hereditary elements in the fertilized egg with the consequent duality of the same ele ments in every cell of the adult body is the most important contribution of modern cytology to the theory of heredity.
While these discoveries were being made during the latter part of the last century, dis coveries that were destined ultimately to throw a flood of light on how inheritance is brought about, a doctrine was attracting widespread attention, but has never succeeded in being brought into systematic relation with our knowledge concerning the cell.
In 1809 Lamarck had advocated the view that evolution is due in part to the direct in fluence of the environment, especially in plants, and in part to the inheritance of structural and physiological changes that first take place in the body of animals as a result of use and dis use of its parts. The doctrine involves the idea that in some way the germ cells respond spe cifically to changes in the body. Darwin ac cepted Lamarck's view (although Darwin erroneously supposed that Lamarck held a view essentially different from his own) and ad vanced in 1866 a tentative hypothesis as to how somatic modifications could affect the germ cells. His view was the first attempt to show how such a process, that had been vague— even mysterious, might be made compatible with the teaching in regard to cell structure and physiology that had at that time made very considerable advances. This is the famous hypothesis of Pangenesis. Darwin suggested that every cell of the body is, at all times, throwing off minute representative particles called gemmules which leave the cell, and, mi grating through the fluids of the body, finally reach the germ cells into which they enter, and which in a sense they construct. It was as sumed that the gemmules from every part of the body unite in the germ-cells with like particles. These particles determine in some way, not clearly specified by the theory, the nature of the organs or characters of the new individual that develops from the egg.