306 GENERAL EMBRYOLOGY 



symmetry, type of cleavage, and the relative positions and 

 properties of future organs being predetermined in the cyto- 

 plasm of the egg cell, while only the differentiations of later 

 development are influenced by the sperm. In short, the egg 

 cytoplasm fixes the type of development and the sperm and egg 

 nuclei supply only the details." And yet we should not over- 

 look this fact, of basic importance, that these fundamental 

 cytoplasmic differentiations have resulted from interactions 

 between the cytoplasm and the nucleus of the oogonial cell, 

 and that the nucleus of the oogonial cell, and egg cell, is itself 

 originally derived in equal parts, from paternal and maternal 

 ancestry. And further many of the conditions of polarity, 

 symmetry, and the like, may in some cases be determined or 

 altered by the entrance and subsequent activity of the spermat- 

 ozoon within the ovum. 



Probably altogether the most striking evidence in support 

 of the hypothesis under consideration is to be found in some 

 of the recent work upon the association of sex with chromosome 

 characters. The nature of this association is so particular and 

 significant that certain chromosomes are actually regarded by 

 many as representing sex " determiners." This relation, 

 besides affording striking evidence in this connection, is of very 

 great importance in itself, and we may therefore consider it at 

 somewhat greater length than this connection alone would 

 justify. 



During recent years many instances have come to light, of a 

 variation in the number of chromosomes in different individuals 

 of a single species. With but very few exceptions these 

 numerical differences are associated with difference in sex, and 

 when any such difference exists it is usually found that the cells 

 of the female contain one or more chromosomes in excess of the 

 number found in the male. In some species then, the chromo- 

 some number may be uneven in one sex, and therefore not all 

 the chromosomes are paired structures. In other cases the 

 equivalent diversity of the chromosome groups is indicated by 

 size differences between the members of a certain pair. 



