108 SCIENCE PROGRESS. 



varieties of protoplasm, that we generally acknowledge the 

 chromatin substance of the nucleus to have some special 

 effect in determining the mode of cleavage, and that we 

 have a far more exact knowledge of the phenomena dis- 

 played by the nucleus during maturation, fecundation, and 

 division than we had at the time when Balfour wrote, does 

 not in any degree invalidate the above statement. The 

 extrusion of the polar bodies and the fusion of the male and 

 female pronuclei are indeed phenomena which are peculiar 

 to the germ cell ; but when the fusion has taken place, all 

 that may be predicated of the oosperm may with equal force 

 be predicated of any other cell in the organism. As the 

 result of impregnation we do not see an altered ovum nor an 

 altered nucleus. We know that, in fact, the ovum has got 

 rid of part of its nuclear substance by the extrusion of the 

 polar bodies, and that the portion thus lost has been made 

 up by the union of the male pronucleus with the remainder 

 of the egg nucleus. We infer that an important conjunc- 

 tion of material has been effected, and the subsequent 

 history of the oosperm and a consideration of the facts of 

 heredity forces us to the conclusion that the union of these 

 minute particles of matter has. in fact, been of the utmost 

 importance ; but further than this we cannot go with cer- 

 tainty ; all beyond is conjecture which has a greater or 

 less amount of probability, according as it enables us to 

 explain, in a reasonable and logical way, a greater or less 

 number of observed facts and sequences. The subsequent 

 history of the oosperm, that is of the ovum after it is 

 impregnated, is an absolute demonstration of epigenesis 

 in the sense in which it was understood by Harvey and 

 by Caspar Friederich Wolff. It is, as has been aptly said 

 by Herbert Spencer, a progression from the simple and 

 homogeneous to the complex and heterogeneous ; there 

 is no unfolding of parts already existent, but a successive 

 formation of new parts, which were not previously existent 

 ■as parts. This is a point which must be insisted upon ; 

 it was insisted upon clearly and distinctly by Wolff in his 

 Theoma Generationis, and his argument stands unshaken 

 to-day as it did when he first wrote it. Epigenesis is a 



