56 REPRODUCTION AND LIFE-HISTORY OF ANIMALS. 



the first without the intervention of the "resting stage," 

 which usually succeeds a nuclear division. The extruded 

 polar bodies come to nothing, though they may linger for a 

 time in the precincts of the ovum, and may even divide. 

 The extrusion of polar globules from mature ova seems to 

 be almost universal among animals ; but observations 

 are lacking in regard to Birds and Reptiles. Moreover, 

 Weismann and Ischikawa have shown that in all partheno- 

 genetic ova which they have examined only one polar body 

 is formed. Blochmann indeed finds that in the eggs which 

 give rise to drone- bees, and in one other case of partheno- 

 genesis, two polar bodies are formed as usual. But in 

 neither of these two exceptional cases is the parthenogenesis 

 habitual; thus many of the eggs which the queen-bee lays are 

 fertilised, and give rise to queens and workers. So far the 

 admitted facts about polar bodies, but their theoretical inter- 

 pretation is much disputed. 



(1) Minot, Balfour, and Van Beneden have suggested that the polar 

 globules represent extrusions of male substance from the egg. The 

 non-formation or retention of one polar body in jjarthenogenetic ova 

 replaces the otherwise necessary sperm, and makes development possible. 



(2) Biitschli, Giard, and others have considered the problem histori- 

 cally, and interpret the premature division of the ovum as a survival 

 of an ancient habit. Just as the mother-sperm-cell divides into units, 

 so the unfertilised ovum divides, though to a less extent. The polar 

 bodies thus represent rudimentary or abortive germ-cells. It is difficult, 

 hovifever, to explain why the habit should so constantly persist. 



(3) Weismann supposes that the two polar bodies are different from 

 one another. The first goes off with a nuclear substance which was 

 only of use when the egg was a-making ; the second effects a quantitative 

 reduction of the more essential nuclear stuff or germ-plasma, thus making 

 room for the addition of a corresponding quantity by the fertilising 

 spermatozoon. Parthenogenetic ova give off only the first kind of 

 substance, and are thus able to divide and develop without the aid of 

 a ^ermatozoon. But there is no evidence that the two polar bodies are 

 different from one another; the fact that the second follows the first 

 without the intervention of a resting stage, suggests the very reverse. 

 Nor does it seem to me probable that the power of developing depends 

 merely upon the presence of a definite quantity of "germ-plasma," 

 retained in parthenogenetic ova, gained in other cases by the addition 

 of a sperm nucleus, which furnishes the equivalent of that which has 

 just been parted with in forming the second polar body. 



(4) A combined theory is that the ovum divides like any other cell, or 

 like its Protozoon ancestors (2), at the limit of growth ; that the extru- 

 sion does in some way differentiate the ovum, perhaps by eliminating 

 waste or male products (i); and that the reduction of the germinal 



