CHAPTER XVI 



THE ROLE OF THE CYTOPLASM IN DEVELOPMENT 

 AND HEREDITY 



In the preceding chapters it has been shown why and how the chromo- 

 somes have been assigned a special role in the development and inherit- 

 ance of the organism's characters. When two individuals develop similar 

 characters under the same environinental conditions, it is concluded that 

 their protoplasmic constitutions are alike, for development is the result 

 of interactions of the protoplasmic sj'stem ^^ith its environment and 

 among its own components. When the interactions and characters are 

 the same in successive generations, the characters are said to be inherited, 

 although this actually means that they have been redeveloped in the 

 offspring because its protoplasm is like that of the parent. When two 

 individuals, whether they are brothers or parent and offspring, develop 

 different characters in the same kind of environment, the differences are 

 attributed to dissimilarities in the constitution of their protoplasms, in 

 particular to differential factors in their chromosomes. It is largely the 

 Mendelian phenomenon and the numerous refinements in experimental 

 procedure that have made this correlation possible. We now face the 

 question, "Are there differential factors elsewhere in the protoplast?" 



Reciprocal Crosses. — A method for ascertaining whether cytoplasmic 

 elements participating in character formation ever act differentially is that 

 of comparing the results of reciprocal crosses. In angiosperms the cyto- 

 plasm is derived mainh- or entirely from the maternal parent. When, 

 therefore, two inbred types of plant are crossed reciprocally, the offspring 

 of the two crosses have nuclei that are alike but cytoplasms that are 

 different. Differences between the two classes of offspring might then 

 be attributed to the differential action of the parental cytoplasms. This 

 is the converse of the situation in crosses employed in most cytogenetic 

 investigation, where the nuclear constitution is made to vary in a uniform 

 cytoplasm. The usual absence of differences between such reciprocal 

 intraspecific hybrids indicates the absence of such cytoplasmic differences 

 between the two types crossed as would lead to differences in character 

 development. In some interspecific and intergeneric hybrids, however, 

 such differences are frequently observable. Often the difference dis- 

 appears in the course of one generation or more. This is interpreted to 

 mean that the egg carries in its cytoplasm a lingering effect impressed 



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