16 INTEODUCTION. 



the ordinary process of cell division ; nor why it is that in the 

 great majority of cases three-fourths of th6 chromatin of the 

 egg nucleus are extruded in the polar bodies. But Weismann's 

 discovery that one polar body is extruded from parthenogenetic 

 eggs is alone sufficient to render revision of the theory necessary. 



Weismann has himself attempted to get over the diflBculty 

 by an elaborate theory which assumes that the two polar bodies 

 are of entirely different nature, and that it is only the second 

 one, the one not usually formed by parthenogenetic eggs, that 

 contains the male elements. The actual mode of formation of 

 the two polar bodies is, however, strongly opposed to the view that 

 there is a fundamental difference between them ; for in all cases 

 that have been carefully observed, the first and second polar 

 bodies are formed in precisely similar manner. But the dis- 

 covery noted above, that, in the cases of the gipsy moth and the 

 drone bee, 6ggs that have extruded two polar bodies can still de- 

 velop parthenogenetically, is fatal to Weismann's theory. 



A more profitable line of inquiry is to compare carefully the 

 phenomena of fertilisation in the Metazoa with those of the lowest 

 animals, and with those of plants. This has been done by 

 Blitschli and Giard, and more recently and in great detail by 

 H'artog. Such a comparison shows that it is a very common 

 occurrence for a primary reproductive cell to give rise, by 

 two or more divisions, to a number of cells of which one alone 

 becomes an ovum, capable of developing into an embryo; 

 while the others serve as accessory organs for the support or 

 nutrition of the ovum, or f6r facilitating the access of sperma- 

 tozoa to it ; or else degenerate and disappear. The formation of 

 polar bodies is probably of similar nature, and is to be regarded 

 as an act of true cell division. By two divisions — for the first 

 polar body frequently, perhaps generally, divides after separation 

 from the egg — the primary reproductive cell, or gonoblast, be- 

 comes divided into four cells, i.e. into three small polar bodies 

 and one large ovum. Each of these four cells contains exactly 

 the same amount of nuclear matter, for this is halved at each 

 division ; and the difference between the ovum and the polar 

 bodies is simply that, as regards the protoplasm of the cell body, 

 the division has been an extremely unequal one; the ovum 

 having appropriated almost the whole of the protoplasm, while 

 the polar bodies possess exceedingly small amounts of this. 



