Section 4. After the Ovum Has Finished Its Growth, and Elaborated the

yolk within itself, a peculiar change occurs in the small area free from yolk-- the animal pole, in which the germinal vesicle lies. This germinal vesicle divides, and one moiety is budded off from the ovum. The ovum has, in fact, undergone cell division into a very large cell containing most of its substance, and a small protoplasmic pimple surrounding half of its nucleus. The disproportion is so great between the two cells, that the phenomenon does not at first suggest the idea of cell division, and it is usually described as the extrusion of the first polar body. There follows a second and similar small cell, behind the first, the second polar body. Since the nucleus of the ovum has divided twice, it is evident that the nucleus remaining now in the ovum is a quarter of the original nucleus. Very little protoplasm is given off with the polar bodies; they play no further part in development, but simply drop off and disappear. Not only in the frog's ovum, but in all vertebrata, two polar bodies are given off in this way before the sexual process occurs. Their exact meaning has been widely discussed. It is fairly evident that some material is removed from the nucleus, which would be detrimental to further developments, and the point debated is what is the precise nature of this excreted material. This burning question we can scarcely deal with here.