HEREDITY l8l 



mission of hereditary qualities in the adult as the egg, although the mass 

 of the latter is, as a rule, many times larger than that of the spermato- 

 zoon, makes it certain that only a small fraction of the contents of the 

 egg has anything to do with this transmission of hereditary qualities. 

 Since the head seems to be the more important part of the spermatozoon 

 for the process of heredity, and this head is a homologue of the egg 

 nucleus, Boveri expressed the idea that the nucleus, and not the proto- 

 plasm, is the really significant part of the egg in matters of heredity. 

 In order to test this idea he undertook a very ingenious experiment; 

 namely, the fertilization of an enucleated fragment of the egg of one 

 species of the sea urchin by the sperm of another species. If his view- 

 were correct, such a hybridization should produce a larva with purely 

 paternal characteristics, as the egg only furnished the protoplasm which 

 was not expected to influence the hereditary quahties. The execution 

 of this experiment is extremely difficult. Boveri is inchned to believe 

 that, according to the experiments carried out so far, the fertilization 

 of enucleated fragments of eggs of Sphar echinus by the sperm of Echi- 

 nus yields plutei of a pure type of Echinus, although he does not consider 

 the question as definitely settled.* I am inclined to beheve that in the 

 early stages the paternal influence would, at the best, be very slight. 



The egg protoplasm contains more or less reserve material which is 

 only gradually transformed into the characteristic compounds of the 

 embryo. It is therefore obvious enough that at first the embryo must 

 show effects of this relation. When the protoplasm of the egg possesses 

 a striking pigment, the larva will possess the same for some time at 

 least ; if such an egg is hybridized with the sperm of a form whose egg 

 is unpigmented, the larva will, of course, possess a "maternal" quaUty 

 which is due solely to the protoplasm (Driesch). In the eggs of birds 

 the incubation period depends ceteris paribus upon the mass of yolk. 

 When a species with a long incubation period is crossed with one of a 

 short incubation period, the egg, and not the sperm, determines the 

 incubation period, as Whitman observed in pigeons. It is obvious, 

 then, that during the first stages of development an influence of the 

 protoplasm upon heredity may make itself felt, which will disappear as 

 soon as the protoplasm of the egg has been transformed into the tissues 

 of the embryo. It does not seem to me that a discussion as to the rela- 

 tive influence of protoplasm and nucleus upon heredity will prove 

 very fertile, but that it is necessary to transfer this problem as soon as 

 possible from the field of histology to that of chemistry or physical 

 chemistry. This view is supported by investigations concerning the 

 toxic effects of blood of one form upon not too closely related forms. 

 * Boveri, Archiv fur Eniwickelungsmechanik, Vol. 2, p. 394, 1896. 



