PREFACE v 



in no way responsible for the philosophical speculations in 

 which I have ventured to indulge. 



The illustrations are largely borrowed from Korschelt and 

 Heider's work, and I must thank Herr Gustav Fischer, of 

 Jena, for his readiness in supplying the blocks. Others are 

 from the original publications l , and I am obliged to the 

 proprietors for permission to make use of them. A few are 

 my own. 



In the appendices will be found an account of some recent 

 work on the relation between the symmetry of the egg and 

 that of the embryo in the Frog, and on the part played by the 

 nucleus in differentiation. 



1 Proceedings of the Boston Society of Natural History, the Journal of 

 Experimental Zoology (Williams & Wilkins, Baltimore), the American 

 Journal of Physiology (Ginn & Co., Boston), Zellen-Studien (Fischer, 

 Jena), Verhandlungen der Anatomischen Geselhchaft (Fischer, Jena), 

 Ergelnisse uber die Konstitution der chromatischen Kernsubstanz (Fischer, 

 Jena), Archiv fur mikroskopische Anatomic (Cohen, Bonn), Archiv fur 

 Entmcklungsmechanik (Engelmann, Leipzig), and the Popular Science 

 Monthly (Appleton & Co., New York). 



