THE GENERAL SCHEME 



body are so closely regulated that parthenogenesis of the 

 sort described by Pincus would not be possible. 



Heredity. We have not yet told the whole story of fertili- 

 zation. Mere stimulation of an egg to develop, necessary as 

 it is, is not all the sperm cell does. It has another vastly 

 important task, which is to carry into the egg the male par- 

 ent's contribution to the heredity of the offspring. Packed in 

 the nucleus of the egg and in the nuclear head of the sperm 

 cell are the submicroscopic chemical particles that control 

 the inherited characteristics of the species; when the sperm 

 cell unites with the egg, the nucleus of the fertilized egg 

 acquires an equal share of this controlling material from 

 each parent. When the egg divides, these determiners are 

 distributed to the daughter cells at each division and thus 

 are carried into all the cells of the embryo. This is the way 

 the sperm brings in "new blood" and rejuvenates the cell 

 lineage, as happens in the one-celled animals by means of 

 conjugation. In this way, moreover, two family lines are 

 blended, and special traits of bodily build and behavior are 

 exchanged and distributed, so that the young are never quite 

 identical with either parent. 



This blending and assortment of hereditary characters is 

 the object and goal of sexual reproduction. Whatever else 

 follows in this book is merely the story of the arrangements 

 and devices of nature to assure the meeting of egg and sperm 

 cell and to protect the embryo that they produce. 



How the determiners of heredity that can shape the whole 

 body of a man or woman, and then bequeath themselves to 

 another generation, are packed into the small compass of 

 egg and sperm cell, how they are distributed to the cells of 

 the body by processes of almost geometrical precision, how 

 they can be traced and how they guide the building of the 

 body — this is the subject matter of the science of genetics, 

 one of the grand divisions of modern biology. It is not to be 

 a theme of the present book. Those of my readers who have 



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