420 INTRODUCTION TO CYTOLOGY 



tial control of genes, these characters, although inherited "maternally" 

 in any particular first-generation hybrid because of the late addition of the 

 male nucleus, should show biparental inheritance through subsequent 

 generations. 



Certain evidence obtained on this point is in accord with these expec- 

 tations. For example, in cross-fertilized sea urchin or fish eggs the rate 

 and the type of cleavage are the same as in the mother, no matter what 

 the direction of the cross; they are characters impressed upon the egg 

 during its ovarian history, and the male nucleus fails to change the 

 condition already induced in the egg by maternal nuclear factors. When 

 the hybrid matures and produces eggs, however, it is found that these 

 all show the dominant rate and type of cleavage, no matter which parent 

 contributes the dominant factor. This shows that the male does affect 

 the character in the second generation. ^^ Breeding experiments with 

 moths and butterflies have given similar results with respect to some 

 embryonic characters. ^^ The conclusion is that many embryonic char- 

 acters whose differentiation is initiated in the egg cytoplasm before 

 syngamy, although fundamentally Mendelian in their inheritance, may 

 be peculiar in that the visible effect of the male gamete is delayed for 

 one generation. 



Conclusions. — The phenomena described in this chapter serve to 

 emphasize the fact that the differential factors responsible for those 

 differences in heritable characters which constitute the materials for 

 genetic study are not all similarly located in the protoplasmic system. 

 In crosses between nearly related types it appears that the differences 

 in character are due mainly if not entirely to differences in the genetic 

 factors in the chromosomes, i.e., to the genes. Such characters show 

 Mendelian inheritance because the genes are distributed in a character- 

 istic manner by the chromosomes through successive life cycles. In 

 crosses between more distantly related organisms there may be, in addi- 

 tion to genie differences, cytoplasmic differences which are sufficiently 

 great and persistent to reveal their influence on the development of 

 hereditary characters. When the cytoplasmic differences are the only 

 ones, as may be the case in reciprocal hybrids, the development of the 

 persistent diversities in character may be said to be "controlled" by 

 these differential cytoplasmic conditions, just as the genes "control" 

 the development of Mendelian characters in narrower crosses. 



It should be recognized that in all cases the development of the 

 inherited characters involves the activity of both nucleus and cytoplasm. 

 The characters are brought to expression through the action of the entire 



13 See Conklin (1915, 1917, 1924), Morgan (1924a/g), Wilson (1925), and Brachet 

 (1927). 



'^ Toyama (1913), Tanaka (1916), Uda (1923). See Morgan et al. (1922) and 

 Morgan (1924a^). 



