176 BIOLOGICAL LECTURES. 



himself would hardly regard it as true in general, but only in 

 respect of such axial organs as would have to shift their posi- 

 tion through an angle of 180° to the opposite side of the body. 

 The morphological inequality of the upper lobes of both 

 human lungs assumed by Aeby (21) — an inequality which he 

 deduces from the fact that the bronchus from the right side 

 takes an eparterial, that of the left, a hyparterial course, so 

 that the left lung lacks an equivalent of the right upper lobe — 

 also rests upon the developmental mechanical assumption that 

 the relations of position of the air-passage to the blood-passage 

 are essentially more constant, i.e., may vary with less facility 

 than the sJiape of the portions of the lung to which these two 

 passages lead. This assumption, though doubtful, is sup- 

 ported by the fact — also of a developmental mechanical 

 nature — that the lung has little shape of its own, but adapts 

 its form largely to its environment. 



The fundamental law propounded by Wiedersheim (22) as 

 the result of extensive comparative investigation, "that the 

 impulse to the development of the appendicular skeleton in 

 vertebrates always starts from the periphery, and that the 

 central (girdle) portions are only secondarily developed under 

 the formative influence of the free appendages," is, as will be 

 seen, also of a purely developmental mechanical nature, and 

 requires further developmental mechanical substantiation and 

 analysis. This is also the case with the important conception 

 of imitative homology, or parhomology, introduced by Fiir- 

 bringer (23). 



Although these examples have been adduced without special 

 selection, they nevertheless show clearly how comparative an- 

 atomy is continually assigning problems to developmental 

 mechanics by making that science acquainted with new opera- 

 tions, and how, on the other hand, developmental mechanics, 

 by devoting itself to the solution of these problems, is becom- 

 ing the continuation and at the same time the mainstay of 

 comparative anatomy. 



As long as comparative anatomy attempted to establish only 

 the main course of development in the animal kingdom, follow- 

 ing in a general way the continuous development of forms only 



