ON THE NUMBER OF POLAR BODIES, &c. 



CONTENTS. 



PAGE 



I. PARTHENOGENETIC AND SEXUAL EGG 339 



The process of the formation of polar bodies very widely distributed . 339 

 The significance of polar bodies according to Minot, Balfour, and van 



Beneden . ... '.1 ...... 340 



- .My hypothesis of the removal of the histogenetic part of the nucleus . 341 



Confirmation by the discovery of polar bodies in parthenogenetic eggs . 345 

 Parthenogenetic eggs form only one polar body, while eggs requiring 



fertilization form two . . . . . . . ' . 346 



Parthenogenesis depends upon the fact that the part of the nucleus 

 which is expelled from sexual eggs in the second polar body, remains 



in the egg 348 



History of this discovery 349 



II. SIGNIFICANCE OF THE SECOND POLAR BODY 352 



Refutation of Minot 's theory . -353 



The second division of the nuclear spindle involves a reduction of the 



ancestral germ-plasms . . . . . . . . 355 



The theoretical necessity for such reduction ...... 356 



Phyletic origin of the germ- plasms in existing species . . . -357 

 The necessary reduction takes place by a special form of nuclear division 358 

 The division which causes this reduction has probably been already 



observed ........... 360 



'Van Beneden's and Carnoy's observations ...... 360 



Two different physiological effects of karyokinesis ..... 364 



Significance of direct nuclear division ....... 365 



Arguments in support of the view that the division of the egg-nucleus 

 which causes reduction must occur at the end of ovogenetic develop- 

 ment . . . . . . . . . . . -367 



Such nuclear division is to be found in the formation of the second polar 



body 368 



History of the origin of this view . . . . . ., 368 



III. THE FOREGOING CONSIDERATIONS APPLIED TO THE MALE GERM-CELLS . 370 



The male germ-cells also require division in order to reduce the ancestral 



germ-plasms . . . . . . . . . . -37 



The germ-plasms of the parents must be contained in the germ-plasm 



of the offspring . 370 



Advantages which the egg gains by the late occurrence of the ' reducing 



division' ........... 371 



The causes of unequal division in the formation of polar bodies . -373 

 These causes do not apply to the sperm-cell ...... 373 



Different kinds of nuclear division occur in spermatogenesis . . . 375 

 Some of these may be interpreted as ' reducing divisions ' 375 



The paranucleus ('Nebenkern') of spermatogenesis probably contains 



the histogenetic nucleoplasm 376 



IV. THE FOREGOING CONSIDERATIONS APPLIED TO PLANTS . . . -377 



Z 



