| | 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] |