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Essays Upon Heredity and Kindred Biological Problems / Authorised Translation cover

Essays Upon Heredity and Kindred Biological Problems / Authorised Translation

Chapter 50: ON THE NUMBER OF POLAR BODIES, &c.
By August Weismann
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About This Book

A collection of linked essays examining biological inheritance and related problems. It opens with an inquiry into factors that determine organismal lifespan and then develops a theory of heredity centered on the continuity of the germ-plasm. Subsequent essays analyze the significance of sexual reproduction, the number and role of polar bodies, and the conditions that allow parthenogenetic development. Other pieces critically evaluate botanical and experimental claims for the transmission of acquired characters and for the heritability of mutilations. Empirical observations are combined with theoretical interpretation, and the essays are presented as successive stages in a progressively refined research program.

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