SYMBIONTICISM IN RELATION TO HEREDITY 117 



nature and behavior of mitochondria and microsymbionts. 

 While a few minor points may be at variance, the theory 

 of Symbionticism appears, in general, to harmonize with 

 the modern conception of heredity. 



The facts that have been estabhshed in connection with 

 mitochondria are few, but to the point. It has been shown 

 by a number of investigators that, in the division of the 

 cell, the mitochondria are more or less equally divided and 

 distributed to the daughter cells (see plate I, fig. 3). Mito- 

 chondria are present in the germ cells. In the ovum they 

 usually retain their characteristic appearance while in the 

 sperm they may coalesce to form the nebenkern and spiral 

 filament. INlitochondria increase by simple fission. The 

 evidence for the bacterial nature of mitochondria has been 

 submitted by the author, and reviewed in a previous chap- 

 ter of this book. Up to the present time, this work has 

 neither been confirmed nor discredited. 



The question arises: What role do the mitochondria 

 play in the mechanism of heredity? The remarkable 

 researches that have been made by Morgan, Davenport, 

 Castle, Calkins, Guyer, Bridges, Shull, Sturtevant, and 

 many others in this country, deVries, Janssens, Johannsen, 

 Bateson, Doncaster, and others in Europe, have fully 

 estabhshed the localization of the heredity transmitting 

 factors in the chromosomes of the nucleus. The history 

 of these researches appear to prove beyond a doubt that 

 the chromosomes are the bearers of heredity. Morgan 

 and his associates, in particular, have been able to corre- 

 late certain characteristics with certain chromosomes. 

 The discovery of the odd or X-chromosome by McGlung 

 ('02) made it possible to correlate definite characters with 

 this X-chromosome. Factors that are located in chromo- 

 somes of this nature have been called sex-Unked by Morgan. 

 By studying variations in the animal or plant, and corre- 

 lating these with the behavior of chromosomes, it has 



