DEVELOPMENTAL MECHANICS. 169 



In the setting to work of the mechanisms of indirect develop- 

 ment lies, however, one of the greatest hindrances to the investi- 

 gation of normal formative modes of direct development. 



In those low organisms in which regeneration steps in 

 promptly after mutilation or after disturbance in the arrange- 

 ment of parts, the value of the experiment is much lessened 

 when it is intended for the investigation of the normal methods 

 of development. On the other hand, the higher organisms are 

 more advantageous in that their regulatory mechanisms, espe- 

 cially during the later stages of development, are much weaker 

 in their manifestation and in part much more difficult to call 

 into activity, i.e., they set in much later after the disturbing 

 influence than they do in lower animals. 



This favorable circumstance enables us to investigate ex- 

 haustibly by means of experiment the processes of normal 

 development in the organisms which rank next to ourselves. 



Owing to the fact that these two typically different kinds of 

 development, as well as the r61e they play in the reactions of 

 animals subjected to experiment, have not been kept distinct 

 heretofore by most experimenters, recent experimental investi- 

 gation has been productive of more confusion than enlight- 

 enment ; quite apart from the fact that the observations 

 themselves leave much to be desired in point of accuracy and 

 completeness, perhaps for the reason that we do not yet suffi- 

 ciently appreciate how much more expenditure of patient obser- 

 vation is required in experimental investigation than in current 

 descriptive embryological investigation. In the latter we are 

 already sufficiently advanced to be able to recognize the differ- 

 ent developmental stages, and we often know when the stage 

 of immediate interest will make its appearance ; whereas un- 

 usual experimental interference may at any time bring forth 

 something new, so that in order to follow up the subject it is 

 often necessary to observe continuously, or at least frequently, 

 by day and night. 



We must not conceal from ourselves the fact that the causal 

 investigation of organisms is one of the most difficult, if not 

 the most difficult, problem which the human intellect has 

 attempted to solve, and that this investigation, like every 



