PACKARD. — INHERITANCE OF ACQUIRED CHARACTERS. 337 



with the simple ordinary larva of Echinus, which he thinks must have 

 been produced chiefly from enucleated fragments. This seemed to be 

 proved by the fact that he could distinguish in a preserved and colored 

 larva whether or not it had originated from a nucleated or enucleated 

 eog by the size of its nuclei, which are considerably smaller in the 

 larva? derived from the enucleated bits of eggs. Hence all doubt 

 seemed removed, and Boveri claims that he has proved that, by cross 

 fertilization of whole eggs or bits of eggs having nuclei, larva? are 

 formed that are half-way between the larval forms of the parent 

 species. On the other hand, larva? arising from the enucleated bits of 

 eggs have entirely the characteristics of the parent (male) species. 

 Hence, if his experiments are correct, he demonstrates the law that 

 the nucleus alone is the bearer of hereditary qualities. Thus the hy- 

 pothesis that the substance of the nucleus of reproductive cells is the 

 physical basis of heredity seems provisionally at least placed on a 

 foundation of fact. 



On the other hand reference should be made to the recent papers of 

 Driesch and of O. Hertwig,* who from researches on the phenomena 

 of cleavage and the formation of organs in the embryo of the frog 

 , regard the egg as isotropic, its first cells as qualitatively alike, the 

 development of the embryo being the result of an epigenetic forma- 

 tion of organs, the process being one of interrelation of the cleavage 

 cells. Hence in place of the mosaic theory of Roux and the germ 

 plasm theory of Weismann, Hertwig substitutes the theory of the 

 controlling inter-adjustments of the embryonic cells and later of the 

 tissues and organs. 



o 



IT. Heredity of Characters acquired during the Life- 

 time of the Individual. 



It would appear that many, if not most, of our leading anatomists 

 and cytologists agree that there may be a physical basis for heredity, 

 and that this basis is afforded by the germ plasm of the nucleus, 

 a portion of which is continuous in succeeding generations. They 

 do however disagree as to whether acquired or " somatogenic" char- 

 acters can be transmitted by heredity, and whether the contents of 

 the nuclei of germ cells are influenced or not by whatever affects the 

 body in general 



* 0. Hertwig, Archiv fur Mikroskop. Anatomie, 22 December, 1893, XLII. 

 662-794. See Abstract by E. A. Andrews in the American Naturalist, March, 

 1892, pp. 272-278. 



vol. xxix. (n. s. xxi.) 22 



