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bronchus, it is obviously difficult to follow their development in a 

 series of embryos. Without a rich material, this is almost impossible 

 if one is to avoid the danger of making mistakes. It also explains 

 the reasons for so many variable pictures in the relationships of 

 the bronchi in the adult tree. 



All of these chief elements are successive productions of the 

 trachea and the stem bronchus by monopody, some, the laterals and 

 medials are produced chiefly at the sides of the end buds, while the 

 remainder composing the dorsal and ventral series are produced up 

 on the stem. They appear to be serial in their origin, however, for 

 I have never observed the formation of a bronchus in an interspace 

 on the stem after the two lateral bronchi limiting it are well formed. 

 If a lateral bronchus is in the process of being formed from a slight 

 evagination on the end bud, the stem above it may produce mono- 

 podially dorsal and ventral elements. The difficulty of the question 

 hitherto has been, that the possibility of suppression, as well as the 

 extreme variabihty of the bronchi in the embryonic stages has not 

 been sufficiently recognized. When, in a certain stage, an element is 

 suppressed, and then it is found in an older embryo, it does not in- 

 dicate necessarily the formation of the branch between these periods 

 unless it occurs at the growing end of the tree. When an element 

 is not present on the stem after the two adjoining lateral bronchi 

 are well formed, it is evident that this portion of the stem would 

 have remained nude throughout the life of the tree. 



Letdig 1857 was, I believe, the first to look upon the lung of 

 higher mammals as a complex of hundreds of simple lungs like those 

 of the frog for example. This view has been widely accepted and 

 one frequently sees the respiratory lobules of the mammalian lung 

 compared to the lungs of lower animals. This, however, is incorrect. 

 The lung of the lower animals represents in mammals simply the stem 

 bronchus and its chief branches. As we ascend the animal scale, new 

 elements are added to the bronchial tree peripherally, which physio- 

 logically, perhaps, may justifiably be compared to the simpler lungs, 

 but are in no sense phylogenetically comparable. Speaking phylo- 

 genetically, we may say that the stem bronchus and its chief branches 

 once possessed a respiratory power, but with the formation of new 

 elements peripheralwards, the respiratory function migrates centri- 

 fugally and the primitive lung becomes transformed into a simple con- 

 ducting apparatus which in the lungs of higher mammals is represented 

 by the stem bronchus and its principal branches. If physiological 

 proof is required for this view, we find, in marsupialia, the stem 



18* 



