286 INTRODUCTION TO CYTOLOGY 



Subsequent researches have shown that the characters in which cross- 

 able organisms may differ and which appear in successive generations 

 according to Mendehan rules may be "transmitted" equally by the two 

 parents, and that their development depends in a special way upon the 

 nuclear organization of the gametes and hence of the zygote (or other 

 cell with which development begins). This analysis of the role of the 

 nucleus has been possible because the nucleus contains a relatively small 

 number of visible bodies, the chromosomes, whose distribution in succes- 

 sive cycles can be readily traced, so that the effects of their behavior can 

 be stated in simple mathematical terms. The success of the method 

 has led some workers to extend the nuclear theory to all heredity, and 

 to consider the cytoplasm as little more than an adaptive, nutritive 

 medium for the chromosomes. 



It is clear that the nucleus does not deserve this monopoly of the 

 responsibility for all hereditary phenomena. Cases are known in which 

 certain characters are obviously due to the constitution of the cytoplasm 

 and may be influenced unequally by the two parents (Chapter XXV). 

 Breeding data indicate clearly a causal connection between chromosomes 

 and Mendelian differences; but since the crosses made must necessarily 

 be narrow, relatively speaking, they yield little evidence as to the basis 

 for the inheritance of those characters which are always the same in the 

 crossed individuals.^ It is to be remembered that in all cases the cyto- 

 plasm is an essential component of the system which undergoes develop- 

 ment and produces the characters; in fact, it is mainly in the extra-nuclear 

 portion of cells that characters are differentiated. The cytoplasm, even 

 though it may be derived from but one parent, obviously is concerned 

 in, and conditions, the reactions which eventuate in inherited characters, 

 and this cytoplasm must not vary in constitution beyond certain limits. 

 Hence the "physical basis of heredity" in a fundamental sense is the 

 whole protoplasmic system concerned in development, although the 

 course of certain developmental reactions and therefore the appearance 

 of certain characters may be correlated with peculiarities in the organiza- 

 tion of the nucleus. The nucleus is not an arbitrary determiner of 

 development: it rather contains a set of conditioning or differential 

 factors which somehow influence in particular ways the developmental 

 processes in the protoplast of which they are an integral part. Evidence 

 for this special influence of the nucleus will now be presented.^ 



1 Cf. Gates (1915a6), Johanssen (1923), and Winkler (1924). 



2 For accounts of Mendelism and the chromosome theory, see Morgan (1919a, 

 1925), Morgan et al. (1922, 1925), Walter (1922), Sirks (1922), Castle (1924), Wilson 

 (1925), D. F. Jones (1925), Sinnott and Dunn (1925), Babcock and Clausen (1927), 

 Goldschmidt (1928a), Stern (1928), Belaf (1928a), Baur (1930), Ekman (1930), Pun- 

 nett (1927), M. Hartmann (1929a), and Sansome and Philp (1932). Matsuura 

 (1929) summarizes researches in plant genetics for 1900-1925. English translations 

 of Mendel's classic paper may be seen in Jour. Roy. Hort. Soc. 26 (1901), Bateson's 



