38 The Unity of tlie Organism 



ness of the motile elements being especially striking in the 

 case of the chicken tick sperm from the fact that the mode 

 of locomotion in this spermatozoon is almost unique. But 

 while the case of mitochondria and other non-nuclear parts 

 of the cell in the development of the spermatozoa, ought to 

 be conclusive that although mitochondria can not be "hered- 

 itary units" in any general sense, they, as well as other 

 cytoplasmic parts, may contribute to the production of 

 hereditary structures, yet it would not be so accepted, prob- 

 ably, by the most exacting theorists because such biologists 

 would not allow that a spermatozoon, being unicellular, can 

 have organs and parts which are subject to heredity "in 

 the strict sense" (i.e. in the sense of the definition of hered- 

 ity set up by these persons). We must, consequently, pro- 

 ceed to the specific task of this section ; namely that of 

 considering what is known about the part played by mito- 

 chondria in the histogenesis of hereditary structures in 

 multi-cellular organisms. 



Nearly all the studies centered upon the question of 

 whether the mitochondria are bearers of heredity have gone 

 on the assumption, quite inadequate according to my view, 

 that the problem is to be solved by ascertaining whether or 

 not the bodies are persistent cell organs, take a definite part 

 in fertilization, and are contributed in equal quantity by 

 the female and male germ-cells. In other words the assump- 

 tion has been that the same criteria which have been relied 

 upon to prove that chromosomes are bearers of heredity, 

 must also be applied for deciding whether or not mitochon- 

 dria have the same office. But numerous studies have also 

 aimed to follow the mitochondria in the genesis of tissues, 

 and herein lies the chief importance of investigations in 

 this domain. Not only have they greatly increased our 

 knowledge about the role played by various parts of the 

 cell in histogenesis, notably of the cytoplasmic parts, but 

 they have put us in possession of much precise information 



