OOGENESIS AND EARLY EMBRYOLOGY ASCARIS 575 



based on conditions in A. megalocephala. This interpretation 

 rests upon the supposition, however, that only the chromosomes 

 carry hereditary quahties. If other portions of the nucleus 

 carry such qualities, we must admit that fertilization can at least 

 begin as soon as the two pronuclei break down to form the 

 'fusion nucleus' or the 'fusion spindle.' The probability of 

 this, at least in the case of A. megalocephala, is somewhat di- 

 minished by the work of Vejdovsky ('12), in which it is shown 

 that from cell generation to cell generation only the chromo- 

 somes persist, not the nuclei. From these chromosomes are 

 derived, not only the new chromosomes, but also the entire new 

 nucleus. Hence all hereditary qualities of the nucleus must 

 necessarily reside within the chromosomes. However, this con- 

 dition is not shown to exist in the case of A. canis, so that nuclear 

 (as distinct from chromosomal) transmission of hereditary char- 

 acters from generation to generation still remains a possibility. 



IVleves ('11) and others have argued that the mitochondria 

 are ' plasma bearers of heredity,' but since the work of Vejdovskj' 

 ('12), Wildman ('12) and Walton ('16 c) has shown that the 

 mitochondria are of nuclear origin, and do not continue from 

 cell generation to cell generation, these bodies may be disregarded 

 in looking for nuclear bearers of hereditary qualities. The 

 works of Boveri ('95, '03) on the fertilization of enucleated egg 

 fragments of sea-urchins, and of Lillie ('12) on Nereis, have 

 brought additional evidence that the mitochondria are not 

 functional as hereditary bearers. In the case of sea-urchins the 

 female mitochondria are present, but apparently do not function. 

 In the case of Nereis the male mitochondria do not even enter 

 the egg, but remain outside in the mid-piece of the sperm. 



Turning now to the purely cytoplasmic structures of the sex 

 cells, we find considerable evidence, at least in the egg, of ability 

 .to carry hereditary traits — mostly physical in nature. The 

 work of Conklin, Wilson, Lefevre and others in the last ten 

 years, has shown that the cytoplasm of the egg may play an im- 

 portant role in the transmission of at least physical traits and 

 characteristics. Organ-forming substances are localized before 

 fertilization, and injury to such a portion prevents the later 



