390 Coe — A Century of Zoology in America. 



aside from its activating effect, only transmits Mendelian 

 characters to the egg. ' ,9 



Further experimental analyses of the nature of the fer- 

 tilization mechanism have recently been made by Mor- 

 gan, Conklin, F. R. Lillie, and others. 



Germinal Localization. — The question as to whether the 

 egg contains localized organ-forming substances has been 

 studied experimentally particularly by means of the cen- 

 trifuge. The results indicate that neither of the older 

 opposing theories of "performation" or ' i epigenesis ' ' is 

 applicable to all eggs, but that in certain organisms the 

 eggs possess a well-marked differentiation while in 

 others each part of the egg is essentially, although prob- 

 ably not absolutely, equip otential. 



The Germplasm Cycle. — Since Weismann's postula- 

 tion of the independence of soma and germplasm in 1885 

 many attempts have been made to trace the path of the 

 hereditary substance from one generation to the next. 

 A recent book by Hegner 10 summarizes the success 

 attained in various groups of animals. 



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 concern- 



J. Loeb, The Organism as a Whole, p. 126, 1916. 



10 The Germ -cell Cycle in Animals, 1914. 



11 The Cell in Development and Inheritance, 1896 ; second edition, 1900. 



