June 28, 1907] 



SCIENCE 



1007 



help us little here, because his so-called ' unit 

 characters ' are only these same superficial 

 individual and specific qualities, and the fact 

 that the crossing of any but comparatively 

 closely related forms is impossible, precludes 

 any final solution along this line. 



The facts at our command, indeed, indicate 

 that the female germ cell determines the fun- 

 damental animal form together with its more 

 constant characters. Drieseh, for example, 

 found that in hybrid echinoderms, no matter 

 from what species the sperm were taken, the 

 manner and rate of cleavage, the character 

 of the mesenchyme formation and of gastru- 

 lation always followed the maternal tjrpe. 

 Standfuss has shown that young butterfly 

 hybrids first resemble the maternal species, 

 but diverge more and more towards the male 

 species with each metamorphosis, as individual 

 qualities begin to assert themselves, until the 

 hybrid comes to its ultimate degree of inter- 

 mediacy. 



If we accept the idea of 'formative sub- 

 stances ' as advanced by Conklin or Lillie, 

 whether we regard these substances as really 

 specific organ-forming materials or as indi- 

 cating corresponding cytoplasmic localizations, 

 then the fundamental organology is already 

 laid down in the egg before the spermatozoon 

 enters, and the egg passes on to cleavage with- 

 out waiting for an equal amount of ' formative 

 substance' to pass out from the male pro- 

 nucleus and take joint possession of the al- 

 ready localized areas. Por example, Lillie' 

 states that at fertilization the presence of the 

 sperm involves no important changes in the 

 topography of the several formative substances 

 which are distributed throughout the cyto- 

 plasm of the egg. Since, then, there is no 

 evidence of such a biparental mechanism in 

 early stages, is it not decidedly far fetched to 

 postulate one, especially when the single one 

 that already exists is adequate? 



If it is true that offspring do not inherit 

 equally from each, but more from the maternal 

 parent, then the argument for the morpho- 

 genetic nature of chromatin based upon the 

 striking parity between the chromosomes of 



Wour. of Exp. Zoel., III., 2, p. 178. 



male and female germ cells after reduction 

 loses its force, if we still cling to the idea 

 that the chromosomes are the exclusive bearers 

 of hereditary qualities. 



Contrary to the view that fertilization is 

 primarily concerned with the mingling of 

 hereditary qualities, many zoologists have 

 come to regard it as satisfying some periodic 

 physiological need of the organism, and while 

 one conception is perhaps as hazy as the other, 

 still the latter opens up an equal possibility 

 that the reduction divisions have to do with 

 maintaining a metabolic equilibrium of some 

 kind, instead of acting as a mechanism for 

 the casting out of heritable morphological 

 units. 



But granted that the chromosomes are the 

 bearers of specific morphogenetic substances, 

 even then the facts (1) that there is apparently 

 a persistence of the individuality of the chro- 

 mosomes through successive cell-generations, 

 and (2) that the final behavior of the chromo- 

 somes in reduction division is in seeming ac- 

 cordance with the demand of the Mendelian 

 principles — even these facts do not necessarily 

 restrict us to a purely chromosomal hypothesis 

 of heredity nor to the idea that offspring in- 

 herit equally from each parent. However, 

 since this is one of the most significant lines 

 of argument for this hypothesis, it merits 

 more detailed discussion. 



Various hybridists, judging from the ex- 

 ternal visible characters of hybrids, came to 

 the conclusion that in the germ cells of hy- 

 brids there must be a separation of parental 

 qualities so that with respect to a given qual- 

 ity, half of the germ cells returned to the 

 maternal, half to the paternal types. Con- 

 versely, I had in the meantime, from studying 

 the germ cells of hybrid doves and pigeons, 

 called attention to the fact that there is ap- 

 parently a segregation of maternal and pa- 

 ternal chromosomes at the reduction period 

 which, if the chromosomes bore hereditary 

 qualities, would lead to the establishment of 

 pure germ cells, and thus afford a possible 

 explanation of how the offspring of hybrids 

 come to show returns to grandparent types.^ 



' Science, February 16, 1900. 



