llS BIOLOGICAL LECTURES. 



of effects, each differentiation tending to become the parent of 

 new differentiations. This conception does not in any manner 

 set aside the necessity of assuming, for each species of animal, 

 a specifically organized germ-plasm, nor does it conflict with the 

 fact that the egg-substance may even show a certain amount 

 of regional differentiation before development begins. It does, 

 however, greatly simplify our view of the germ-plasm, and 

 removes it in a measure from that inaccessible and mysterious 

 resfion where Weismann and his followers would place it. 

 These cases reveal, furthermore, the vital part played in de- 

 velopment by environmental conditions. We perceive that our 

 attention has been focused so closely upon the germ-plasm re- 

 garded as the substratum of inheritance and development as to 

 obscure our view of the essential relation in which it stands to 

 the conditions under which development takes place. In other 

 words our point of view has been too largely morphological 

 while the physiological aspect of development has been thrown 

 into the background. 



Recent writers in embryology, foremost among them Driesch, 

 Hertwig, Loeb, and Herbst, have, however, clearly perceived 

 the vital importance of the conditions of development in dis- 

 tinction to the innate tendencies of the germ-plasm, and 

 Driesch especially has published an elaborate theory of devel- 

 opment in which full recognition is accorded to the physiologi- 

 cal aspect of the question. The germ of this theory is con- 

 tained in the writings of Wilhelm His, and the views of this 

 profound and philosophical embryologist are of immense im- 

 portance because of the abundant fruit they have borne in the 

 works of the experimental morphologists. His resolves the 

 work of the ontogeny into two factors. The first of these is 

 the "law of growth" (Wachsthumsgesetz) that is inherent in 

 the germ-plasm or idioplasm (Keimstoff) and represents the 

 essential element of inheritance ; the second comprises the 

 conditions under which that "law" operates (such as the 

 shape and size of the egg, distribution of yolk, pressure of 

 membranes, nature of the surrounding medium, and the like). 

 Every event in the development may therefore be conceived as 

 the product of these two factors. Thus, the various foldings 



