CHAPTER XVI 



THE EMBRYOLOGICAL AND CYTOLOOICAL EVI- 

 DENCE THAT THE CHROMOSOMES ARE THE 

 BEARERS OF THE HEREDITARY UNITS 



Long before the genetic evidence brought forward its 

 abundant data that are explicable on the theory that the 

 chromosomes carry the genes, embryologists had already 

 found other evidence that led them to regard the chromo- 

 somes as the bearers of the hereditary factors. Taken 

 as a whole, this evidence makes out a very strong case 

 for the chroniosomes, but since it did not establish the 

 relation beyond question, the genetic evidence was all 

 the more welcome. 



The earliest evidence, sometimes cited in favor of 

 chromosomal inheritance, was based on the statements 

 that in some cases at least, only the head of the spermato- 

 zoon enters the egg. Since it was then thought that the 

 head is composed almost entirely of the nucleus, and since 

 the child inherits equally (in the older parlance) from its 

 father and from its mother, it followed that the nucleus 

 carries the hereditary elements. When later it became 

 known that the head of the sperm represents almost 

 exclusively the mass of condensed chromatin, it was sup- 

 posed that the chromosomes, in particular, must be that 

 part of the nucleus that is the bearer of hereditary charac- 

 ters. Such a conclusion received indirect support from 

 the facts, then becoming known, that the chromosomes 

 remain constant through successive generations of cells, 

 whereas the nuclear sap becomes lost in the gen- 

 eral cytoplasm each time that the nuclear wall is dis- 

 solved. It was also found that the spindle fibres disappear 

 in the resting stages, while the nuclear reticulum (chro- 

 matin) remains. 



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