Now, in certain cases of parthenogenesis, it has been found that one of the polar bodies may not be given off, but, remaining in the egg, its nucleus reunites with the egg nucleus, and thus takes the place of the spermatozoon, which does exactly the same thing when it fertilizes the egg, _i.e._ the nucleus of the spermatozoon unites with the nucleus of the egg. This fact in regard to the action of the polar body in fertilization is not as surprising as appears at first sight, for if each of the polar bodies is equivalent to a spermatozoon, the fertilization of the egg by one of its own polar bodies conforms to theory. There is a considerable body of evidence showing that in many eggs at one of the two maturation divisions the chromatin rods derived from the nucleus are divided crosswise (Fig. 6 B, C). The same thing occurs at one of the two divisions in the formation of the spermatozoon (Fig. 7 B, C). At the other division to form the other polar body (or the other sperm-cell) the chromatin rods appear to be split lengthwise, as in ordinary cell division (Fig. 6 E, F, G). In recent years the _cross-division_ of the chromatin rods has attracted a great deal of notice, and Weismann in particular drew attention to the possible importance of this kind of division. There is another fact that gives this division especial significance. It has been discovered that the number of chromosomes that appears in each dividing cell of the organism is a constant number, but it has also been discovered that the egg, before extruding its polar bodies, and the mother-cell of the spermatozoon (Figs. 6, 7 B), contain exactly half of the number of chromosomes that are characteristic of the body-cells of the same animal (Figs. 6, 7 A). Now there is good evidence to show that the reduction in number is due to the chromosomes uniting sometimes end to end in pairs, as shown in Figures A and B. Furthermore, it has been suggested that at one of the maturation divisions, when the chromosomes divide crosswise, the united chromosomes are separated (Figs. 6, 7 B, C), so that one remains in the egg and the other goes out into the polar body. The same thing is supposed to occur at one of the maturation divisions of the sperm mother-cell. A further consideration of capital importance in this connection has been advocated by Montgomery and by Sutton, namely, that, when the chromosomes unite in pairs, a chromosome from one parent unites with one from the other parent. Consequently at one of the two reduction divisions maternal and paternal chromosomes may separate again, some to go to one cell, some to the other.