422 E. L. SHAFFER. 



last maturation division and ever afterward would not justify 

 us in assuming that this is a growth period in the history of the 

 spermatozoon; but rather that it is a transformation period of 

 those organs that are already present and fully-grown in the 

 spermatid stage. This transformation in all the organs of the 

 cell is merely an elongation such as could be brought about, 

 doubtless, by prolonged lateral pressure." 



D. GENERAL CONSIDERATIONS. 



The importance of the mitochondria as bearers of hereditary 

 units rests on their mode of origin and maintenance through 

 the cell cycle, their behavior in fertilization and their role in 

 differentiation. If, as has been previously discussed, the view 

 is correct that the .mitochondria owe their origin to materials 

 derived from the nucleus or by the activity of the nucleus, then 

 their importance in heredity can only be secondary. 



Just how much of the spermatozoon enters the egg is a matter 

 of importance in ascertaining the role of the mitochondria. In 

 Nereis, Lillie (1912) has found that the middle piece and the 

 tail of the spermatozoon do not enter the egg. On the other 

 hand, Meves (1911) has shown that the entire spermatozoon 

 enters the egg of Ascaris, and Van der Stricht (1909) has shown 

 similar results in the bat. It therefore seems impossible to make 

 any generalizations on this subject until more work on the de- 

 tails of fertilization has been done. In the case of Peripatus, 

 Montgomery (1912) has shown that the mitochondria are en- 

 tirely lost in the spermatozoon, being thrown off. within certain 

 cytoplasmic lobes. Here, at least, the mitochondria of the 

 spermatozoon can play no part in the transmission of hereditary 

 characters. 



The mitochondria have been most exhaustively studied in 

 somatic cells where they are present in a variety of forms. 

 Certain workers (Meves, Duesberg, Hoven and others) have 

 maintained that the mitochondria give rise to myofibrils, neuro- 

 fibrils and other somatic differentiations; but these views have 

 not been strongly substantiated. The work of Cowdry (1914) is 

 strong evidence that the mitochondria of nerve cells are not 

 transformed into neurofibrils. 



