Nothing impresses the stamp of truth upon an hypothesis more than the fact that its light renders intelligible not only those facts for the explanation of which it has been framed, but also other and more distantly related groups of phenomena. This seems to me to be the case with my hypothesis, since the interpretation of polar bodies and the ideas derived from it unite from very different points of view, the facts of reproduction, heredity and even the transformation of species, into a comprehensive system, which although by no means complete, is nevertheless harmonious, and therefore satisfactory.
Only the most essential elements of the new facts which form the foundation of the views developed in this essay will be briefly mentioned. My object is to show all the theoretical bearings of these new facts, not to describe them in technical detail. Such a description accompanied by the necessary figures will shortly be given in another place[[234]].
A. W.
Freiburg I. Br., May 30, 1887.
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 | [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] |