EMBRYOLOGY 



101 



character of the ancestral forms to which the several em- 

 bryonic stages correspond. 



The phenomena of homology are as evident in the study 

 of embryology as in anatomy. Many structures in the 

 embryo can be properly understood only after comparison 

 with similar organs in other forms to which they are related. 



Another class of structures, which we may call nascent 

 organs, appears in the embryology of very many forms. 

 These are organs which 

 begin to appear during 

 the development of the 

 animal or plant, but 

 which never become 

 fully developed or nor- 

 mally functional, and 

 soon disappear before 

 the adult condition is 

 reached. They recall 

 some ancestral condition 

 in which these organs 

 were important, and are 



Of interest aS Showing B . The single aperture (mouth anus) by which the 



. , i i i i digestive cavity opens to the exterior. 



the racial history, but, 



so far as we now can judge, the weakly developed rudiments 

 of these structures are of little importance to their present 

 possessors. Numerous examples might be given. I will 

 mention but one. 



The jellyfishes and their relatives have but a single open- 

 ing into their alimentary canal, which serves both for the inges- 

 tion of food and the egestion of wastes (Fig. 22). Most of the 



Hydra. A diagrammatic longitudinal 



