The eggs of a sea urchin, Sphærechinus, which normally develop into a well-known larva, were broken by Boveri into two in such a way that the nucleus was all contained in one fragment. Male cells of another species of sea urchin, Echinus, having a well-known, but quite different, larva, he then caused to enter the fragment without a nucleus. A larva developed which possessed all the characters of the larva normal to the male used in the experiment. Since cell contents of both species were present and nuclear structures of only one, and the larva resembled the species represented by a nucleus, it was concluded that the hereditary substance is located in the nucleus. The nucleus, then, contains the physical basis of heredity.
Boveri's results were looked upon with much suspicion until they were confirmed by Delage. With a German and a Frenchman agreeing we may safely consider this point as settled.
The intimate process of the preparation of the hereditary cells for their union and the union of the cells have been the subjects of many monographs during the last twenty years.
Since each cell has a definite number of chromosomes and this number would be doubled by the union of two cells elaborate provisions are made by the cells to reduce this number to one half before the union of the two cells takes place. The study of the methods of this reduction has engaged a host of cytologists during the past ten years, and innumerable papers have resulted.
It is evident that the great difference between the two cells is simply an adaptation to insure their union, for, after uniting, their nuclei, the physical basis of heredity, become alike (c in figure 11).
Credit for the great activity in research along this line must be