94 THE MITOCHONDRIAL CONSTITUENTS OF PROTOPLASM. 



IX. EMBRYOLOGY. 



FERTILIZATION. 



Investigations on the behavior of mitochondria during the process of fertil- 

 ization are at the root of the theory that they constitute the cytoplasmic basis of 

 heredity. These studies have brought to light several facts: 



(1) That in the multiplication of spermatogonia and spermatocytes in the 

 testis, the mitochondria are divided in approximately equal amounts between the 

 daughter-cells. 



The mitochondria are usually divided en masse. There seems to be no gen- 

 eral provision for a qualitatively equal division of the mitochondrial substance in 

 any waj' analogous to the longitudinal splitting of the chromosomes. Wilson's 

 (1916, p. 539) discovery of the pecuUar behavior of the mitochondria in the scorpion 

 must be mentioned. He found that in the Arizona scorpion there is in spermatogen- 

 esis an accurate fjualitative distribution of mitochondrial substance. The mito- 

 chondria become condensed into a ring of material which "divides somewhat after 

 the fashion of a heterotype chromosome ring, each spermatid receiving exactly one- 

 fourth of its substance." In the Cahfornia scorpion, on the other hand, the process 

 is entirely different. "The ring is here absent, its place being taken by about 

 twenty-four separate hollow spheroidal bodies that show no evidence of division 

 at any time and establish no definite relation to the spindle, but are passively 

 segregated by the spermatocyte divisions into four approximately equal groups. 

 Each spermatid thus receives as a rule sLx, not uncommonly five, rarely seven of 

 these bodies." Now, the astonishing regularity of the process in the Arizona 

 scorpion recalls to mind the behavior of the chromosomes themselves and strongly 

 suggests that we are here deahng with a distribution of substances which may ac- 

 tually play a part in heredity. But the phenomena observed in the California scor- 

 pion lead irresistibly to a diametricallj' opposed conclusion, that the mitochondria 

 are not carriers of heredity, since their distribution to the spermatids seems to take 

 place in an irregular, haphazard way. One can not help remarking how fortunate 

 it is that the two forms were studied at once, because otherwise it would be diffi- 

 cult indeed to resist being led astray by the extraordinary behavior of the mito- 

 chondria in the first of them. 



(2) That structures derived from mitochondria persist in the fully developed 

 spermatozoon. * 



Benda (1897, p. 401) was the first to show that they become transformed into 

 the spiral filament of the sperm. This observation has been corroborated by many 

 authors. The mitochondria also form the "nebenkern." But the mitochondrial 

 substance is usually confined to the middle piece of the sperm. Here also the great 

 variations in the behavior of the mitochondrial material are recorded, and we notice 

 at once the absence of anytliing approximating the remarkable orderUness which 

 is so characteristic of the changes which the chromatin undergoes. Finally, the 



