406 



SCIENCE 



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



tions have been made. For the others it 

 would obviously have been too enormous a 

 task to have been profitable. The races from 

 zygospore germinations have been added as 

 being likely to show through segregation 

 sexual abnormalities if such existed. Nearly 

 10,000 combinations have been made using 

 nearly 2,000 different races of diverse types 

 *• of mucors and no race of a diecious species 

 has been found which, if it showed any sex- 

 uality at all, reacted other than as a plus or 

 a minus. 



We have just been discussing infra-specific 

 sexual reactions. The next table shows inter- 

 specific reactions previously discussed under 

 the term " imperfect hybridization." In test- 

 ing the reactions between the plus and minus 

 races of two different species, all the four 

 possible interspecific combinations have been 

 made but, since the combinations between 

 races with like signs have never given re- 

 actions, they have been omitted from the 

 table. Only a -part of the possible combina- 

 tions have yet been tested, but sufficient to 

 indicate that the same sexual dimorphism ex- 

 ists in all the species investigated. 



We feel justified in concluding from our 

 experience, that the forms in the tables are 

 sexually dimorphic. From our experience with 

 the diecious sporophytes of willows and pop- 

 lars, such a strict dimorphism was hardly to 

 have been expected. It would be a safe wager 

 that one could not examine even a hundred 

 individuals of either of these genera without 

 finding sex intergrades. The apparent sharper 

 differentiation of sex in the diecious mucors 

 in comparison with higher plants is perhaps 

 connected with the fact that in mucors we 

 are dealing with sexually differentiated game- 

 tophytes instead of with sporophytes. 



GAMETE DIFFERENTIATION 



I should like to close our discussion by a 

 consideration of gamete difierentiation in 

 mucors and other forms. As a general rule, 

 all of the diecious mucors represented at the 

 top of the chart (Fig. 4) have gametes equal 

 in size. Of the hermaphrodites there are two 

 types — those with equal gametes (isogamic), 



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 («a"5EXE5l)ll (+. \%\ 



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Fig. 



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CANETES EOUAL 



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UCOIII- BMKOI IWD fliniESaiT SUDAL RUCTIWS 

 SOLID LMES RETOCSENI SUCCESTED CDU85E 

 OfCAMnEOirrERENTIATIIHI 

 4. Diagram illustrating sexual reaetioM 



ind gamete differentiation. 



figure below, and those with a constant and 

 marked difference in size (heterogamic), fig- 

 ures at right and left. We can conceive of 

 the hermaphrodites as having been derived 

 from the diecious types or the diecious types 

 from the hermaphrodites. If the latter be 

 the actual course of evolution, we may con- 

 ceive a differentiation of sex to have taken 

 place in two directions beginning with the 

 isogamic hermaphrodites — first toward a differ- 

 entiation, chiefly physiological, separating the 

 sexes on separate plus and minus individuals 

 in diecious forms; second toward a differ- 

 entiation, conspicuously morphological, bring- 

 ing about a constant difference in the size of 

 the gametes in the heterogamic hermaphro- 

 dites. The prevalent biological distinction 

 between males and females is based ultimately 

 upon the relative size of the gametes which 

 they produce. The smaller gamete is con- 

 sidered the male; the larger recognized in the 

 left figure by the outgrowths behind it, is con- 

 sidered the female. The diagram (6) shows 

 the reactions obtained in attempting, by use 

 of this criterion of sex, to homologize the plus 

 and minus signs with the terms male and 

 female or vice versa. The hermaphrodite, 

 which is heterogamic, is grown between the 

 plus and minus races of Mucor V. On the 

 right its smaller gamete reacts with the plus 



