Coe — A Century of Zoology in America. 391 



ing the relation between the chromosomes and sex. 

 Besides Wilson, Montgomery, Mark, McClung, Morgan, 

 Miss Stevens, Conklin and their associates and students 

 have now furnished conclusive evidence that the sex of 

 an organism is determined by, or associated with, the 

 nuclear constitution of the fertilized egg. This consti- 

 tution is moreover shown to be dependent upon the chro- 

 mosomes received from the germ cells. 



This explanation is in strict accordance with the results 

 of experimental breeding. It is also quite in harmony 

 with the Mendelian law of inheritance, and in fact forms 

 one of the strongest supports for the view that all Men- 

 delian factors are resident in the chromosomes. Recent 

 work has also discovered the mechanism which governs 

 the complicated conditions of sex which occur in those 

 animals which exhibit alternating sexual and partheno- 

 genetic generations. These remarkable processes are in 

 all cases found to depend upon a definite distribution of 

 the chromosomes. 



Other recent experimental work has shown that while 

 the sex is thus normally determined in the fertilized egg, 

 it is in some animals not irrevocably fixed, and the normal 

 effect of the sex chromosomes may be inhibited by 

 abnormal conditions in the developing embryo, as is 

 demonstrated by the recent work of Lillie and others. 



The cytological basis for Mendelian inheritance has 

 been very extensively studied by Morgan and his pupils 

 in connection with their work on inheritance in the com- 

 mon fruit fly Drosophila. The evidence supports AVeis- 

 mann's earlier hypothesis that the chromosomes are the 

 bearers of the heritable factors, and that these are 

 arranged in a series in the different chromosomes. This 

 theory is shown to be in such strict accord with both the 

 cytological studies and the results of experimental breed- 

 ing that Morgan has ventured to indicate definite points 

 in particular chromosomes as the loci of definite heri- 

 table factors, or genes. 



Confirmation of this view is furnished by the behavior 

 of the so-called sex-linked characters, the genes for which 

 are situated in the same chromosome as that which 

 carries the sex factor. Many ingenious breeding experi- 

 ments indicate further that all the hereditary characters 

 in Drosophila are borne in four great linkage groups 



