THE MITOCHONDRIAL CONSTITUENTS OF PROTOPLASM. 97 



pass into the egg on fertilization are exceptional and that the crucial cases are those 

 in which no mitochondrial substance passes into the egg. This Lillie believes to 

 be the case in Nereis. Mitochondria generally occur in the middle piece and tail 

 of the spermatozoon, though this is not always true. Lilhe (1912, p. 418) says 

 that "the middle piece and tail of the spermatozoon do not enter in the fertiUza- 

 tion of Nereis." He admits (p. 426) that "it is possible that the fixation granules 

 produced by the spermatozoon represent a cytoplasmic element." So that, until 

 new facts are discovered, through the use of mitochondrial methods of technique, 

 the case of Nereis does not offer an insurmountable barrier to the acceptance of 

 the view that mitochondria play a part in inheritance. 



MITOCHONDRIAL CONTINUITY. 



The question of mitochondrial continuity arises just as surely as the doctrine 

 of the permanence of the chromosomes, and the proof of it is every bit as unsatis- 

 factory, perhaps more so. 



Duesberg (1912, p. 766) is in favor of the theory of mitochondrial continuity; 

 Beckwith (1914, p. 230), Chambers (1915, p. 291), and several others are against 

 it. It has taken strong hold on the botanists, GuilUermond (1912a, p. 398) com- 

 ing out strongly in favor of it and saying: "The mitochondria result from the 

 division of the preexisting mitochondria of the egg, none of them ever arise 'de 

 novo' in the cytoplasm." However, in certain plants some mitochondria are 

 large and others extraordinarily minute. It is possible that the large ones may 

 arise in this way by division of preexisting ones, but there is no evidence that the 

 small ones are formed only by the segmentation of filaments or rods, which are of 

 larger girth than they are. They probably arise de novo by condensation. 



The idea that all mitochondria arise from preexisting mitochondria by divi- 

 sion is a relic of Altmann's (1894, p. 155) doctrine, Omne granulum ex granule, and 

 has persisted in our minds on account of the newly conceived idea that mitochondria 

 are concerned in heredity. Altmann thought that they were elementary organisms 

 endowed with a certain measure of individuaUty. It is possible that the truly 

 remarkable morphological resemblance which they bear to bacteria led him to 

 believe that they multiply in this way. This conclusion has been supported quite 

 recently by the growing tendency to regard mitochondria as plast-like in nature. 

 It is apparently quite true that mitochondria form plastids in some plants, and, 

 in view of the evidence at hand that the plasts exhibit a true genetic continuity, 

 some of them always arising from preexisting plastids by division, it was quite 

 natural to assume that the mitochondria themselves behaved in somewhat the same 

 way. 



It has been generally believed for some time past that mitochondria multiply 

 by transverse division and perhaps also by longitudinal division. The evidence 

 for the latter method is meager and unsatisfactory and does not merit discussion. 

 With regard to transverse division, it must be said that this undoubtedly does 

 occur in some cases — for example, in the course of spermatogenesis of Vespa crabro, 

 as was found by Meves and Duesberg (1908, fig. 39). This has already been men- 



