THE MORPHOLOGY OF FERTILIZATION 77 



onic stages; such characters are therefore exclusively 

 maternal. The materials of the cytoplasm are, how- 

 ever, being constantly consumed in the metabolism, 

 and the process of renewal and increase of such materials 

 involves interaction of nucleus and cytoplasm; there- 

 fore the purely maternal cytoplasm soon disappears 

 and is replaced by cytoplasm formed under the influence 

 of the biparental zygote nucleus. Maternal cytoplasmic 

 characters cannot therefore survive long in the life- 

 history, unless the cytoplasm contains elements either 

 that survive as such or that increase independently of 

 the nucleus. Mitochondria granules may be such ele- 

 ments, and in plants also plastids, including the chloro- 

 phyll grains of chloroplasts. There may be conceivably 

 other chemical substances that have a purely cyto- 

 plasmic history, but for this we have little evidence. 

 We have, however, adequate cytological grounds in 

 such persistent elements of composition of the egg 

 cytoplasm for the explanation of the rather rare cases 

 of exclusively maternal inheritance from a zygote 

 known to the geneticists. Purely paternal inheritance 

 probably does not exist in any regularly formed zygote, 

 and this constitutes an independent line of negative 

 evidence against cytoplasmic inheritance from the male 

 side. 



VI. POLYSPERMY 



The ova so far considered are normally monospermic ; 

 there are, however, certain ova into which more than 

 one spermatozoon enters normally, and practically all 

 ova may become polyspermic under abnormal condi- 

 tions. We may thus distinguish normal, or physio- 

 logical, and pathological polyspermy. 



