THE NATURAL HISTORY OF MAN 49 



formation of gill clefts but soon disappear. Their presence, 

 however, is indicative of a fish stage in his development. The 

 lungs, which are the permanent respiratory organs (Fig. 19) 

 of man, are developed from the upper end of the alimentary 

 canal by the formation of a single hollow sac. This later 



Frog 



Fig. 17. The individual and evolutionary development of the nerve cell. 

 After Cajal. Redrawn from Donaldson, The Growth of the Brain. 



bifurcates to form the rudiments of the right and left lungs and 

 these by repeated branching develop the highly ramified, tubu- 

 lar structure of the adult lungs. The early simple, sac-like 

 lung of the human embryo is similar in structure to the per- 

 manent, saccular lung of the adult amphibians. 



From the neck region of the alimentary tube, three other 

 organs develop as branching outgrowths, all of which finally 

 form organs of internal secretion, namely the thyroid, the 



