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bronchus and its chief branches have the power of carrying on respir- 

 atory processes, for, as Selenka 1887 and, more particularly, Bremer 

 1904 have shown, in Didelphys virginiana, by the time the young are 

 transferred to the pouch, only the stem and the principal bronchi are 

 already formed. These are forced, according to Bremer, to carry on 

 the respiration of the organism until the respiratory elements used in 

 the adult stages are produced. 



There is also other evidence for this view to which attention has 

 not been called which lies in the method of growth. According to 

 the older ideas, the differentiation of the amphibian and reptilian lung 

 took place by an ingrowth of septa from the periphery of the lung, 

 essentially a centripetal process. It was also well known that the stem 

 bronchus and chief branches of the higher vertebrate lung were pro- 

 duced by a process of centrifugal growth, monopodial in nature ac- 

 cording to some authors and dichotomous according to others, but, in 

 both instances, centrifugal. Here the ontogeny and phylogeny of the 

 vertebrate lung stood in apparent conflict. Recently the work of 

 Moser 1902, Schmalhausen 1905, and particularly the thorough stu- 

 dies of Hesser 1905 have shown that the main divisions of the am- 

 phibian, reptilian, and avian lung are produced by monopodial growth. 

 We are thus now able to study the development of the mammalian 

 lung from a new view point, finding in the researches of these authors 

 a phylogenetic confirmation for our views on the monopodial growth 

 of the stem bronchus and its chief branches. Simple budding from 

 the walls of the stem would seem to be the most primitive type of 

 division, but there is no essential difi"erence in the process whether it 

 takes place from the terminus of the bronchus or from a point higher 

 up on the stem. Hesser has shown beautifully in reconstructions of 

 the reptilian lung how the budding takes place from the cephalic end 

 of the simple lung and proceeds downwards, a process which, of course, 

 is recapitulated in the successive production of branches on the stem 

 bronchus in mammals. 



After the formation of the chief branches has occurred, the primi- 

 tive monopodial system may persist for a few generations on the side 

 branches. The principal method of division is, however, by dichotomy 

 equal and unequal. Apparently the selection of the method depends 

 somewhat on the physical conditions of the space in which the bronchi 

 are forced to divide. In the case of the first divisions of Lateral 1, 

 of Lateral 2 on each side, and Ventral 2 on the right side, the division 

 is of practically equal dichotomy, as they have a relatively free space 

 about them. When, however, the direction is more or less controlled 



