A CENTURY OF ZOOLOGY IN AMERICA 429 



Cytology. 



Another important field of investigation which has 

 attracted many workers is that which pertains to the life 

 of the cell the science of cytology. Although the cell- 

 theory was established as early as 1839, little advance 

 was made in this subject in America before 1880. Since 

 that time, however, Americans have been so successful in 

 cytological discoveries that they are now among the 

 world's leaders in this field. 



These studies have been followed along both descrip- 

 tive and experimental lines. The most prominent of the 

 early workers in this field are E. L. Mark and E. B. Wil- 

 son. Mark's description of the maturation, fecundation, 

 and segmentation of the egg is the most accurate and 

 complete of the early cytological studies. Wilson's 

 discoveries concerning the details of fertilization and 

 his " Atlas of Fertilization and Karyokinesis, " pub- 

 lished in 1895, have now become classic. Wilson, too, 

 has published the only American text-book on cytology, 11 

 and has more recently taken the lead in studies con- 

 cerning 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 parthenp- 

 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> 



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