!68 BIOLOGICAL LECTURES. 



in very small parts, so that, therefore, areas of independent 

 differentiation may at an early stage comprise a single or only 

 a few cells. The investigation of such narrowly localized 

 processes of differentiation is attended with much greater 

 difficulties ; and since, moreover, the fundamental formative 

 processes, viz., assimilation, growth, self-movement, and the 

 qualitative differentiation of cells take place altogether or, at 

 least, in the first instance within the province of the invisibly 

 minute, it will be necessary, in order to clear up these funda- 

 mental processes, to make as much or even more use of hy- 

 potheses, as physicists and chemists are compelled to do when 

 they cope with the fundamental processes of their respective 

 sciences. And just as in these sciences, we shall have to 

 regard those assumptions as approximating most nearly to the 

 truth which explain the most facts and permit of the successful 

 prediction of new facts ; and ceteris paribus we shall prefer that 

 explanation which appears to be the " simplest," not forgetting, 

 however, that we may easily fall into error on this point for the 

 reasons above set forth. 



Experiment on living beings is quite peculiar and apt to be 

 misleading, in that in many cases, like mutilations and certain 

 disturbances of the arrangement of parts with respect to one 

 another, conditions arise in which the organism does not react 

 with the formative mechanisms of direct or normal development, 

 but with the regulative and regenerative mechanisms of indirect 

 development, or regeneration (13). 



Indirect development runs its course in great measure under 

 the regulating reciprocal activities of many, or, as in the case 

 of great defects and disturbances in lower animals, for a time at 

 least, of all parts of the organism ; it differs essentially in this 

 respect from the direct or typical development of the fertilized 

 ovum, which goes on in the absence of any interference, or 

 even for a short time after the cessation of the interference, 

 and often completes its course with extreme self -differentiation 

 of circumscribed parts. ( Within these, of course, the changes 

 depend on the reciprocal operations of the parts?} 



The modi operandi of each of these two varieties of develop- 

 ment must be investigated. 



