4 The Development of the Lungs 
Between these structures a transverse gutter appears, while the portion 
of the head gut anterior to this, produced by the narrowing of its lateral 
walls, forms a longitudinally placed laryngo-tracheal groove, which gives 
rise to the trachea and larynx. The separation from the cesophagus 
then begins at the caudal extremity and proceeds forwards. 
Among the earlier investigators there was an apparent unanimity of 
opinion that the subsequent differentiation of the amphibian and reptil- 
ian lung was due to a centripetal ingrowth of septa from the lung wall 
dividing and subdividing the primitive lung cavity into a series of smaller 
peripheral spaces. Furthermore, as early as the middle of the last century 
Leydig, 57, taught that the complicated lungs of the higher vertebrates 
represented a complex of a series of simpler lungs, or, in other words, 
that the infundibulum of the mammalian lung might be compared with 
an entire frog’s lung with its parietal alveoli. Miller, 93, in a compara- 
tive study of the reptilian, avian, and mammalian lung, states that the 
complexity of the reptilian lung is due to a system of septum formation 
while the process of budding plays a secondary réle. In the avian lung, 
however, budding becomes more important and septum formation is 
secondary. Thus Miller looked upon the avian lung as a transition stage 
between the reptilian lung with its septum formation and the mammalian 
lung produced by the budding process. 
In an extensive study of the dried lungs of adult reptiles Milani, 
94, 97, emphasizes the importance of septum formation for the differen- 
tiation of the pulmonary apparatus as one ascends the animal scale. The 
formation and enlargement of primary septa upon the dorsal and ventral 
walls of the lung cavity which extend horizontally from the median to 
the lateral wall of the lung as well as the further subdivision of these 
spaces by secondary septa is responsible for the gradual evolution of the 
complex from the simple lung. 
Ever since the work of Kolliker, 79, the architecture of the mammalian 
lung has unanimously been conceeded by all who have worked upon the 
embryonic stages to rest upon a process of centrifugal budding. The 
centripetal formation of septa, apparently, plays no part in its evolution. 
There has been, therefore, a great gap between the developmental pro- 
cesses in the reptilian, amphibian, and avian lung, on the one hand, and 
the mammalian lung on the other, for, as Gegenbaur has pointed out, 
ontogeny and phylogeny have apparently stood in conflict, as the pul- 
monary apparatus in the ancestors of the mammals was produced by a 
process exactly opposite to that which ontogeny shows is responsible for 
the growth of the mammalian lung. 
The first work which has offered us a suitable explanation of this 
