294 THE EVOLUTION THEORY 



Auorbiicli, Sclinoidor, ami Biitschli, had seen stages oi' the process at 

 an earlier date Avithout arriving at the true interpretation of the 

 phenomena. This was chiefly due to the fact that, in addition to the 

 phenomena of fertilization proper, which we have Imefly sketched, 

 other nuclear changes take place in the maturing ovum, and these are 

 not very easy to distinguish from the former ; we refer to the pheno- 

 mena of the so-called ' maturation of the ovum.' When the ovum-cell 

 lias attained its full size within the ovary it is not yet capable of being- 

 fertilized, but must first undergo two processes of division, to the 

 right understanding of which Hertwig's investigations, and afterwards 

 those of Fol, have contributed much. 



For a long time it had been a familiar observation that small 

 refractive corpuscles were extruded from one pole of the ovum shortly 

 before the beginning of embryonic development. These were called 

 ' polar bodies,' because it was believed that they marked the place 

 which would afterwards be intersected by the first plane of division ; 

 it was only known at that time that they had to be extruded from the 

 Ggg, but no one had the remotest idea of their real nature. 



We now know that they are cells, and that their origin depends 

 on a twice repeated division of the egg-cell : but it is a very unequal 

 division, for these ' directive cells ' or ' polar bodies ' are always much 

 smaller than the ovum, and indeed are usually so small that it is easy 

 to understand why their cellular nature was for so long overlooked. 

 Yet they have always a cell-body, and in many ova, for instance those 

 of certain marine Nudibranchs, this is quite considerable ; and they 

 have likewise always a nucleus, which, notwithstanding the smallness 

 of the cell-body, is in all cases exactly of the same size as the sister 

 nucleus which remains behind in the ovum after division — a fact 

 which is in itself enough to indicate that we have here to do 

 essentially with readjustments and changes in the nucleus of the 

 ovum. 



Long before the polar or directive divisions were recognized as 

 divisions of the egg-cell it was known that the nucleus of the ovum 

 disappeared as soon as the latter attained to its full size within the 

 ovary. It was also known that this nucleus— the large so-called 

 ' germinal vesicle ' lying in the middle of the ovum — left its central 

 position and moved to the upper surface of the ovum, there to become 

 paler and paler, and ultimately to disappear altogether from the sight 

 of the observer. By many it was believed that it broke up, and that 

 the 'segmentation nucleus,' which is afterwards obvious, is a new 

 formation. The truth is that the germinal vesicle, at the time of its 

 disappearance, is transformed into a division fip-ure which is invisiltle 



