Mechanism of Mendelian Heredity 245 



no physiological or chemical connection between 

 them, and second : How can a factor contained in the 

 chromosome determine a hereditary character of the 

 organism? To the first question we venture to offer 

 the answer which has been already suggested in various 

 chapters of this book, that the cytoplasm of the egg 

 is the future embryo in the rough ; and that the factors 

 of heredity in the sperm only act by impressing the 

 details upon the rough block. This metaphor will 

 receive a more definite meaning by the answer to the 

 second question. The characters which follow Mende- 

 lian heredity are morphological features as well as 

 instincts. For the former we have already had occa- 

 sion to show in previous chapters to what extent they 

 depend upon the internal secretions or the existence of 

 specific compounds in the circulation, and the same is 

 true for the instincts (Chapters VIII and X). This 

 then leads us to the suggestion that these determiners 

 contained in the chromosomes give rise each to the 

 formation of one or more specific substances which 

 influence various parts of the body. We probably 

 do not notice all the effects in each case, but when a 

 special organ is affected in a conspicuous way, we con- 

 nect the factor with this organ or the special feature of 

 the organ which is altered, and speak of a determiner or 

 factor for that organ, or for one of its characters. We 

 also understand in this way why outside conditions 

 should be able to overcome the hereditary tendency 



