SOME GENERAL CONCEPTS IN EMBRYOLOGY 11 



The normal embryology of the frog is well understood, and there is 

 no better form through which to introduce the student to the science 

 of embryology. The frog is a representative vertebrate, having both 

 an aquatic and a terrestrial existence during its development. This 

 metamorphosis requires, for instance, several major changes in the 

 respiratory and excretory systems during development. The embry- 

 ology of the frog illustrates the sequence of developmental events 

 (organogeny) of all higher vertebrates. Finally, the living embryos of 

 the frog are now available at any season of the year and students can 

 observe directly the day-to-day changes that occur from fertilization to 

 metamorphosis. 



Some General Concepts in Embryology 



Biogenesis 



There is no substantiated evidence of spontaneous generation of 

 life, although, as a scientist, one cannot deny its possibility. All life 

 does come from preexisting life. In embryology we are more specific 

 and state that all organisms come from eggs. This statement is cer- 

 tainly true of all Vertebrates and of the vast majority of animals. How- 

 ever, it cannot be said of the single-celled Protozoa or of some of the 

 lower Invertebrates which reproduce by binary fission, spore forma- 

 tion, or budding. 



All protoplasm in existence today is believed to be descended from 

 preexisting protoplasm and it is therefore related by a continuous line 

 from the original protoplasmic mass, whenever that came into ex- 

 istence. Further, sexual organisms living today are each descendants 

 of a continuous line of ancestors not one of which failed to reach the 

 period of sexual productivity. So, the mere existence of an organism 

 today is testimony of its basic relationship to all protoplasm and to its 

 inherited tendency toward relative longevity. At present one cannot 

 conceive of life originating in any manner but from life itself. All 

 theories are, of necessity, philosophic speculation. 



Biogenetic Law — The Law of Recapitulation 



The original laws of von Baer are very clearly stated, but Haeckel 

 and others have misinterpreted or elaborated on them so that a con- 

 fusion about these concepts has arisen in the popular mind. Von Baer 

 clearly emphasized the fundamental similarity of certain early stages 



