408 



SCIENCE 



[N. S. Vol. LI. No. 1321 



plasms in the mucors, we may be able to spur 

 our imagination still further to conceive of 

 this same constitution as existing in one of 

 the two sexes in all organic forms. It might 

 then be theoretically possible by proper tech- 

 nique to obtain reactions with our isogamic 

 plus and minus races of the mucors and thus 

 have males and females in different groups of 

 plants and animals compared on a common 

 and fimdamental basis. If this highly imag- 

 inative proceedure were possible, is there any 

 reason to believe that the so-called males in all 

 groups of plants and animals would invariably 

 be related to the same sex — plus or minus of 

 the mucors? It might transpire that the so- 

 called females of the moths and birds, to 

 take an extreme example, would be foimd by 

 their reactions with test mucors to bear the 

 same sign — plus or minus — as the males of 

 flies and marmnals. 



Sex has apparently developed independently 

 many times in different groups of plants and 

 animals. The term male and female are ap- 

 plied to the end products seen in visibly 

 dimorphic gametes. There is no assurance 

 that these terms have laid hold of the funda- 

 mental differences between the two sexes. 

 Spines, superficially similar, are develoi)ed on 

 the i>orcupine, jimsonweed and sea urchin, 

 yet these have no close genetic relationship to 

 one another. They are examples of parallel 

 development in imrelated structures — in other 

 words they are to be considered analogous 

 rather than homologous organs. Is it not 

 possible that visible differences in dimorphic 

 gametes are also analogous rather than homo- 

 logous; that the sperm in one form may be 

 homologous to the egg in another form? It 

 is suggestive in this connection that the males 

 of mammals have this in common with the 

 females of birds — that they produce two kinds 

 of gametes. Moreover, it is the sex glands 

 of the male of mammals and of the female 

 of birds which form hormones influencing 

 profoundly the expression of the secondary 

 sex characters, albeit in a somewhat different 

 manner. I do not suggest that in starting 

 with human terminology, as we generally 

 have done in describing lower organisms, we 



should call the rooster a female and the hen 

 a male. I wish merely to call attention to 

 inadequately explained sexual phenomena in 

 higher forms in which similarities in the 

 gross morphological differentiation of the so- 

 called male gametes of two forms are not 

 associated with certain physiological peculiar- 

 ities which are common rather to the opposite 

 sex^. 



It seems reasonable to consider in mucors 

 the physiological sexual differentiation into 

 plus and minus races, more expressive of any 

 fundamental peculiarities of sex, if such 

 actually exist, than the size differences and 

 associated phenomena in higher forms. Sperm 

 cells, in addition to being gametes, are organs 

 of locomotion and the egg cells, in addition 

 to being gametes, are storage cells to supply 

 nourishment to the developing zygote. Motil- 

 ity in the sperm and storage in the egg we 

 can conceive of as secondary rather than 

 primary sex characters. It is not alone the 

 gametes of higher forms in which we find 

 differences associated with the diverse func- 

 tions of bringing the gametes together and 

 nourishing the zygote formed by their union, 

 but also the two sexual organisms themselves 

 may have their sexual differences related 

 directly or indirectly to these same somewhat 

 conflicting fimctions. 



The diecious mucors seem largely free from 

 such secondary sexual characters which may 

 tend to obscure more fundamental sexual 

 differences. Their gametes are normally equal 

 in size and nourishment for the developing 

 zygote is supplied approximately in equal 

 amoimts from both sexes. Moreover, in those 

 few forms in which the conjugative filaments 

 seem to exercise attraction toward each other, 

 such attractions seem to be mutual and equal. 



It would carry us too far to attempt to 

 meet the objections of cytologists or of others 

 to our hypothesis of gamete differentiation or 

 to attempt to show in what other ways the 

 sexual differentiation in mucors may be of 

 interest to students of higher forms. We will 

 be satisfied, however, if we have shown that 

 the simple bread mold may eventually be of 

 some service in helping to solve the funds- 



