Joseph Marshall Flint 103 
the head gut as a ventral groove with a more marked projection at the 
caudal extremity, which becomes separated from the dorsal segment of 
the gut by two longitudinal fissures, along the line of which the final 
separation occurs. The upper part of the anlage gives rise to the trachea, 
the lower to the lungs. If the pulmonary apparatus in mammals should 
finally be shown to have a serial relationship with the gill pouches, all 
trace of the process is certainly lost in the pig. From the first, the anlage 
is asymmetrical. Whether this is a characteristic of the respiratory appa- 
ratus or is due, as Minot suggests, to the influence of the heart, it is 
impossible, from my material, to say. Suggestive, however, is the fact 
that the pulmonary anlage in many of the lower animals is symmetrical. 
THE GROWTH OF THE BRONCHIAL TREE. 
Few of the many characteristics of the bronchial tree have given rise 
to more discussion than the method of its growth. Between the two 
extremes of dichotomy and monopody, most of the possible intermediate 
processes have been described. A special review of the literature on this 
point seems desirable to see what harmony can be drawn from the differ- 
ent observations. So far as possible when space permits, the process will 
be described in the words of the various contributors to this field. 
If we recapitulate the history of the several series of bronchi it may be 
said that all of the chief bronchi are produced in the same manner, that 
is to say by monopodial growth. Even the formation of the stem bronchi 
from the pulmonary anlage does not differ in any material way from the 
subsequent formation of the products of the stems themselves. As the 
tree grows, there is no definite division of the end bud as the main 
branches are outgrowths of the walls of the trachea or the two stem 
bronchi. In the pig, the trachea produces only a single element, namely, 
Lateral 1. The process of growth is successive, that is to say, the ele- 
ments are produced one after another from above downwards, recapi- 
tulating the manner of growth shown in simpler animals like the reptiles, 
for example. When a new element is about to be formed, one notes an 
increase in the number of karyokinetic figures in the epithelium in the 
region of the new branch. The basement membrane becomes much less 
distinct and the connective-tissue nuclei in the surrounding mesoderm 
are more closely packed together. In this region, a slight bulging of the 
epithelial wall is then noted, as is shown, for example, in Fig. 12, which 
increases in size until a small elevation is raised on the surface of the 
stem. This subsequently grows, yielding a rounded projection on the 
stem, which gradually emancipates itself and gives rise to a new bronchus. 
