Within the nucleus is a substance distinguishable from the rest; it is distributed in granules and is called the chromatin (1). When the cell is about to divide this chromatin becomes arranged in a long coiled thread (2), and then (3) this chromatic thread breaks into short rods called chromosomes. Two little granules now appear, one at each end of the nucleus, and very delicate threads, the asters, appear to pass from each of these bodies towards the chromosomes (4). Each of the latter then splits lengthways into two, and a half chromosome appears to be drawn by the asters towards the poles of the nucleus. The latter then divides (5) and then the whole cell divides. What thus, in essence, happens in nuclear divisions is that the chromatin of the nucleus is more or less accurately halved. Apparently this substance consists of very minute granules and the whole process is directed towards the splitting of each of these granules into two. A half-granule then goes to each of the daughter nuclei. Every time the embryo divides this process is repeated. Thus each of the (theoretically) 1028 cells of the blastula contains 1/1028th of the substance of each chromatic granule in the fertilised ovum. Pfluger and Roux (in 1883 and 1888 respectively) were the pioneers in the experimental study of the development of the ovum, and the results of their work and that of their successors has, more than anything else in biology, modified and shaped our notions of the activities of the organism. Roux found, or thought so at least, that the first division of the frog’s egg marked out the right and left halves of the body, the one blastomere giving rise to the right half, the other to the left half. The next division, which separates each of these blastomeres, marked out the anterior and posterior parts of the embryo. Thus:-- [Illustration: FIG. 14.--The frog’s egg in the 4-blastomere stage seen from the top.] Now in an experiment which has become classical Roux succeeded in killing one of the blastomeres in the 2-cell stage, while the other remained alive. The uninjured blastomere then continued to develop, _but it gave rise to a half-embryo only_.