154 Animal Life and Intelligence. 



ovum-nucleus. For the ovum, besides being a germ-bearer, 

 is a specialized cell, and its special form is determined by 

 the body-plasm it contains. This is got rid of in the first 

 polar cell, and nothing but germ-plasm remains. Now, 

 if nothing further took place, all the ova of this same 

 individual containing similar germ-plasm would be identical, 

 and similarly with all the sperms from the same parent. 

 The union of these similar ova from one parent with similar 

 sperms from another should therefore give rise to similar 

 offspring. But the offspring are not all similar; they 

 vary. Professor Weismann here makes use of the second 

 polar cell.* "A reduction of the germ-plasm," he says, 

 " is brought about by its formation, a reduction not only 

 in quantity, but above all, in the complexity of its constitu- 

 tion. By means of the second nuclear division, the excessive 

 accumulation of different kinds of hereditary tendencies or 

 germ-plasms is prevented. With the nucleus of the second 

 polar body, as many different kinds of plasm are removed 

 from the egg as will be afterwards introduced by the sperm- 

 nucleus." " If, therefore, every egg expels half the number 

 of its ancestral germ-plasms during maturation, the germ- 

 cells of the same mother cannot contain the same hereditary 

 tendencies, unless we make the supposition that corre- 

 sponding ancestral germ-plasms chance to be retained by 

 all eggs a supposition that cannot be sustained." 



The two polar cells are therefore, on this view, of totally 

 different character ; and the nuclear division in each case 

 of a special kind and sui generis. I do not think that the 

 evidence afforded by observation lends much support to 

 this view. But with that we are not here specially con- 

 cerned. We have to consider how this reduction of the 

 number of ancestral germ-plasms can further the kind of 

 variation required. Now, it is difficult to see, and Professor 

 Weismann does not explain, how the getting rid of certain 

 ancestral tendencies can give rise to new characters or the 

 enhancement of old characters. One can understand how 

 this "reducing division," as Dr. Weismann calls it, can 



* Weismaun, Essays on Heredity," pp. 355, 78. 



