Joseph Marshall Flint 9 
ratus appears. The latter is formed from two asymmetrical thickenings 
of the lateral wall of the head gut, the left of which appears first in an 
embryo with 18 primitive vertebrae a little- below and ventralwards to 
the last trace of the branchial crest. A constructive process, which these 
authors hypothecate, isolates the entire ventral segment of the head gut 
carrying with it the rudimentary lungs and extending as far cephalad 
as the last gill pouches. Weber and Buvignier obviously abandon the 
idea of the primitive unpaired anlage described by Kolliker, Uskow, and 
His, and with it the conception of a pulmonary groove formed syncro- 
nously with or before the lungs. Thus, the trachea is post-pulmonary in 
origin and is formed by this constructive process involving the ventral 
part of the head gut in the region behind the gill pouches. Like Gotte 
in Anura, Kastschenko in the chick, and Fol in man, Weber and Buvig- 
nier look upon the pulmonary apparatus as diverticule of the head gut 
serially homologous with gill puoches. 
Very briefly Blisnianskaja, 04, describes the paihes of the human 
lung as a projection in the ventral portion of the foregut, which, in an 
embryo of 4.5 mm., shows by two lateral grooves the beginning separa- 
tion of the respiratory from the digestive system. At this stage, how- 
ever, the two systems are still in open communication. 
It is apparent that here is a practical unanimity of opinion among 
those who have contributed to our knowledge of the development of the 
mammalian lung as to the nature of the anlage and the process by which 
the primitive lung sacs are produced. Slight differences of opinion may 
be explained by the nature of the material and the methods by which 
the different observers have worked. Fol, who believes in a_ paired 
anlage for the human lung, studied an embryo somewhat older than the 
specimens of His and Blsnianskaja, while Weber and Buvignier and 
Willach, with this single exception, stand alone in regarding mammalian 
respiratory apparatus as arising from primitively paired structures. In 
turning, on the other hand, to the consideration of the organogenetic pro- 
cesses by which the bronchial tree is produced not only are few authors in 
accord, but, also, there is scarcely a chapter in the whole of embryology in 
which we find so many different opinions based apparently upon objective 
work. It will be wise, therefore, to consider briefly first the results which 
have been obtained by the different contributors to this chapter on the 
development of the lungs, and then attempt to make therefrom a fair 
statement of our knowledge of the architecture and origin of the bronchial 
tree at the present time. 
Before the appearance of Aeby’s paper we had no general conceptions 
concerning the architecture of the bronchial tree. According to the cur- 
