xiv TRANSLATORS INTRODUCTION 



even to the extent that in this treatise Hertwig 



o 



nimself insists, upon the points of agreement 

 between the two views. We are only at the 

 beginning of inquiry into the problems of heredity, 

 and the protagonists of the opposing views, like 

 all those who care more for knowledge than for 

 argument, are concerned more for truth than for 

 the establishment of a modus vivendi. Reconcilia- 

 tion is the parent of slothful thinking and of 

 glosses ; it is by sharp contrasting of the opposing 

 views that we are like to have new facts elicited, 

 and new lines of inquiry suggested. 



As many are interested in the problems who 



have little acquaintance with the technical facts 



of embryology, a simple account of the early 



stages in the development of an animal may be 



useful for reference. I shall choose back-boned 



animals, as, from the inclusion of man among them, 



they are of more general interest. The process 



begins with the fertilisation of the egg- cell by the 



fusion with its nucleus of the nucleus or head of a 



male-cell or spermatozoon. At their first origin 



the nuclei of the sperm and of the egg may be 



of very different appearance, while that of the 



sperm is invariably smaller than that of the egg. 



But before or during the process of fertilisation, 



changes take place, the result of which is that 



the fusing nuclei are exactly alike in morphological 



character. The chromatin, or peculiar substance 



of the nuclei, is transformed into a number of 



bodies known as chromosomes, which are of the 



same number, form, and size, in the two sexes. 



