TEXT-BOOK OF 

 GENERAL EMBRYOLOGY 



CHAPTER I 



ONTOGENY 



LIVING organisms come into existence only as the offspring 

 of preexisting living organisms of the same kind or species. 

 Aristotle's belief that eels were generated from mud and slime 

 is represented to-day, in many youthful minds, by the firm 

 conviction that a horse hair, if only kept long enough in water, 

 will surely "turn to a worm." From the time of Redi the 

 belief that the living might be generated from the wholly 

 non-living gradually became restricted, in its application, to 

 lower and still lower groups of organisms. For a long time it 

 remained applied only to those forms at the lower limit of the 

 living the bacteria. From this position the belief that " spon- 

 taneous generation" of organisms occurs nowadays, was 

 finally driven by the brilliant demonstrations of Pasteur and 

 Tyndall that even these simplest and smallest of organisms 

 arise only from preexisting living organisms of the same kind. 



This property of producing new, specifically similar individ- 

 uals is one of the few really distinctive characteristics of living 

 things, and since the newly produced resemble closely the 

 parent form, we speak of the property as Reproduction. The 

 fact that at corresponding ages offspring resemble their parents, 

 is the fact of heredity. But when these offspring are first 

 distinguishable as separate and new individuals they bear little 

 or no visible resemblance to the adult organisms producing 

 them. This resemblance appears gradually, as the result of a 

 long series of processes, complex and often very special, in- 

 volving changes in structure, function, and form, only at the 

 conclusion of which has the new organism reproduced, more 



1 



