Lefevre — The Advance of Zoology in the Nineteenth Century. 103 



cell determines how it is ffoinor to react to external forces and 

 stimuli, and how it is going to be combined in a definite man- 

 ner with other cells. Two fertilized egg-cdls, subjected to 

 precisely the same external conditions, react in- utterly differ- 

 ent ways. A hen's Q^g and a duck's egg, tying side by side 

 in the same incubator, or under the same hen, give rise one 

 to a fowl and the other to a duck. The difference must be 

 attributed to a difference in the nature of the cell itself, and 

 whether we shall ever understand what lies behind this differ- 

 ence remains ;for the future to decide. We can, however, 

 guard against the delusion that we have an explanation where 

 none exists. Yet so brilliant have been the achievements 

 within the past few years in the fields of cytological and ex- 

 perimental research, that we can set no limit on the possible 

 advance in our knowledge of inheritance and development. 



IV. EXPERIMENTAL MORPHOLOGY, 



Finally, this outline of the advance of zoology in the cen- 

 tury would be incomplete without a reference to a very recent 

 development of the science which has been brought about by 

 the application of the experimental method to the investiga- 

 tion of fundamental problems of heredity, development and 

 growth. In the field of embryological development, the in- 

 vestigation has been pushed with the greatest enthusiasm, and 

 by subjecting the Qgg to entirely new conditions, which can 

 be altered and controlled by experiment, illuminating and 

 far-reaching results have been obtained. Not only has the 

 Qgg been thus experimentally studied, but the same method 

 has been applied to all developmental stages from the time of 

 fertilization onward. It has been entirely through work of 

 this nature that we have arrived at an epigenetic conception 

 of development. 



Again, much attention has been directed to the study of 

 the regeneration or replacement of lost parts by growth in 

 animals and plants, an investigation which obviously lends 

 itself well to the experimental method, in the hope of dis- 

 covering the causes underlymg this remarkable power of 

 living things. Although the study is only in its first stages 



