190 GENERAL BIOLOGY 



essentially one of " restitution," yet occasionally 

 the formative forces get off the morphogenetic track, 

 and a wholly abnormal structure results. In certain 



Crustacea, such as 

 the crayfish, if the 

 experimenter cuts 

 off one of the eye- 

 stalks, another will 

 grow to replace it, 



but if, in addition 



^ , 



X /^ V* to severing the eye- 



stalk, he also de- 

 \ \ stroys the deeper- 



^ lying ganglion, the 



FIG. 70. Regeneration of antenna-like resultant growth 

 organ in place of eye-stalk, in Palcemon. *. ctnlL- 



(From Morgan, after Herbst.) 



at all, but an 



antenna-like structure. (See fig. 70.) It is inter- 

 esting to find that the new organ is still of the crus- 

 tacean type. Such a diversion of the normal path 

 of differentiation is termed heteromorphosis. 



Theories of Morphogenesis. - - The nature of the 

 changes involved in the ontogeny of both plants 

 and animals has been briefly outlined. The ques- 

 tion arises : Why does the germ always give rise 

 to precisely the type characteristic of the species 

 and no other? Nothing that our microscope can 

 tell us of the young embryo within the eggshell 

 of a pigeon or a sparrow gives us the slightest clue 

 as to why the culmination of the development of one 



