REJUVENESCENCE IN EMBRYO AND LARVA 409 



require fertilization for their development can be induced experi- 

 mentally to develop without fertilization. General agreement has 

 not yet been reached as to the nature of the changes concerned in 

 the initiation of development, but there can be no doubt that the 

 increased metabolic activity which in nature follows fertilization 

 may be brought about by the action of certain experimental con- 

 ditions. A great variety of agents and conditions have been em- 

 ployed in these experiments. Harvey ('10) has tabulated the 

 different methods. A few of these methods bring about in certain 

 species a normal, orderly development like that which occurs after 

 fertilization. With many of the so-called parthenogenic agents, 

 however, and in some species with all, the changes which are initi- 

 ated differ more or less widely from normal development. In some 

 cases development may proceed more or less normally through 

 the earlier stages, but ends in death at or before a certain stage; 

 in others the forms produced are clearly abnormal from the begin- 

 ning; in still others only a few divisions, or only changes in the 

 membrane, occur before death. In certain cases also some degree 

 of differentiation without any cell division results from the use of 

 these agents. 



All of these experimental effects have very commonly been 

 regarded as initiation of development, but if the term " develop- 

 ment" means anything, it means an orderly series of events leading 

 to a certain definite result. The course of events and the result 

 attained are subject to more or less variation, and it is not always 

 possible to make a sharp distinction between what is and what is 

 not development. Nevertheless, it is evident that many of these 

 experimental treatments of the egg do not initiate development, 

 but a change which lacks some of the essential features of develop- 

 ment and soon leads to death. To maintain that any experimental 

 agent or condition which brings about some degree or kind of 

 cellular activity in the egg initiates development is to lose sight 

 entirely of the fundamental characteristics of development; and to 

 use such experimental data indiscriminately as a basis for con- 

 clusions concerning the nature of fertilization is certainly not a 

 justifiable procedure. It cannot be doubted, however, that devel- 

 opment in the strictest sense is initiated experimentally in certain 



