224 PHYSICAL BASIS OF HEEEDITY 



bring in the characteristics that make the organism first 

 a bilateral one, then a vertebrate, then a mammal, and, 

 lastly, a perissodactyl. In all these aspects, both parents 

 agree, and beyond these limits hybridizing is impossible. 

 Whatever the germ develops into must contain these com- 

 mon characters. The important point to determine, 

 Boveri thought, is whether the species characteristics are 

 or are not in the nucleus. He concluded, after discussing 

 the pros and cons, that it is doubtful if these preformed 

 qualities of the egg-protoplasm extend beyond the larval 

 periods, but that in general all characteristics that distin- 

 guish the individual from aU others of its species and 

 from the characteristics of related species are inherited 

 through the chromosomes. Later he restated his con- 

 clusion as follows: *^A11 essential characteristics of the 

 individual and of the species are epigenetic, and the deter- 

 mination is brought about through the nucleus. ^ ' Conklin 

 at one time expressed even more sharply the idea that 

 group characteristics may be inherited in a different way 

 from specific characters in the following paragraph: 



We are vertebrates because our mothers were vertebrates and pro- 

 duced eggs of the vertebrate pattern; but the color of our skin and 

 hair and eyes, our sex, stature, and mental peculiarities were determined 

 by the sperm as well as by the egg from which we came. There is 

 evidence that the chromosomes of the egg and sperm are the seat of 

 the differential factors or determiners for Mendelian characters, while 

 the general polarity, symmetry and pattern of the embryo are deter- 

 mined by the cytoplasm of the egg. 



In another statement, however, Conklin takes what 

 seems to me to be more nearly a correct view in regard to 

 the question, viz., that ' * There is no doubt that most of the 

 differentiations of the egg cytoplasm have arisen during 

 the ovarian history of the egg, and as a result of the 

 interaction of nucleus and cytoplasm ; but the fact remains 

 that at the time of fertilization the hereditary potencies 

 of the two germ-cells are not equal, all the early stages of 

 development, including the polarity, symmetry, type of 



