| 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 | 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 development | 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 | 370 | |
| 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 |
| V. | Conclusions As Regards Heredity | 378 |
| The germ-cell of an individual contains an unequal combination of hereditary tendencies | 378 | |
| Dissimilarity between the offspring of the same parents | 379 | |
| Identity of twins produced from a single egg | 380 | |
| VI. | Recapitulation | 383 |