76 PROBLEMS OF FERTILIZATION 



in one of the animal or one of the vegetative quartet; 

 he even traced it to the thirty-two-celled stage. Thus 

 it is not broken up and distributed to all of the cells, 

 as the theory that it represents a substratum for bearing 

 heredity factors would require, nor does it exhibit any 

 signs of activity. In the egg of an ascidian (Phallusia) 

 the same author (1913) could follow the sperm plasto- 

 chondria through part of the fertilization stages, but 

 then lost sight of them. Van der Stricht, in the bat, 

 and Lams, in the guinea-pig, found that the tail of the 

 spermatozoon and connecting piece which carries plasto- 

 chondria pass into one only of the first two cells. 

 Finally, in Nereis, according to my own observations, 

 the middle piece of the spermatozoon, which is usually 

 supposed to carry the plastochondria, does not enter 

 the egg at all. 



Whatever may be the function of the mitochondria 

 in cell physiology it must be admitted that the study 

 of fertilization has shown no reason for the assumption 

 that their introduction into the egg by the sperm is 

 necessary for the transmission of paternal character- 

 istics. The variable quantity in different cases and the 

 distribution to single blastomeres in certain cases 

 exclude the hypothesis that they have any specific 

 paternal hereditary effect. There is no reason to deny 

 that sperm mitochondria function in the egg when 

 present, but if so it is probable that they are not dif- 

 ferentiated in their chemical composition or genetic 

 behavior from the mitochondria of the egg itself. 



3. The egg cytoplasm. — The egg cytoplasm and its 

 inclusions constitute an exclusively maternal material 

 which determines many of the characters of early embry- 



