29 
is then noted, which increases until a small elevation is raised upon 
the surface of the stem. This increases in size, yielding a rounded 
projection, which gradually emancipates itself and gives rise to a new 
bronchus. The process is essentially the same whether it occurs in the 
neighborhood of the terminal bud or higher up on the stem. In general, 
we may say, the lateral and medial elements are produced nearer the 
terminal end of the main bronchus, while the dorsal and ventral elements 
are produced nearer the terminal end of the main bronchus, while the 
dorsal and ventral elements are formed somewhat higher up, often 
where*the stem has regained its cylindrical form. 
Subsequent division of the branches may occur either by mono- 
pody or dichotomy. Often monopodial production of buds persists for 
one or two generations on the main bronchi, then the method becomes 
dichotomous, either equal or unequal in nature depending somewhat 
on the space in which the bronchi have to divide. In the case of 
equal division of the bud, however, one fork grows on to become the 
stem while the other remains as the side branch. The first division 
of the main bronchi may, it is well to note, be dichotomous as in the 
case of Lateral 1 and Lateral 2. Thus in its growth, the mammalian 
lung recapitulates the history of the simpler lungs of lower animals. 
11. The pulmonary arteries in the pig arise from the pulmonary 
arches as Bremer has described. At first, they run parallel, then bend 
towards each other, sending out anastomoses, which yield finally a 
common trunk with two origins above and two arteries below. Later 
the upper part of the right artery degenerates and with it the right 
pulmonary arch. At 5 mm before the pulmonary arteries may be 
followed as far as the anlage of the lungs, the pulmonary vein may be 
seen as a slight ingrowth from the undivided portion of the Sinus 
venosus, passing through the Mesocardium posterior towards the pul- 
monary anlage. If forms almost in the medial plane. With this es- 
tablishment of the venous outlet ventralwards to the anlage, the ar- 
teries, as the growth of the organ, proceeds, are naturally developed 
from the capillary plexus on the dorsal side of the primitive bronchi. 
This fixes the position of the arteries with reference to the stem bronchi 
before any of the side branches are produced. As the pulmonary an- 
lage projects come distance ventralwards from the head gut, Lateral 1, 
the “eparterial” bronchus, develops above and behind the artery, while 
Lateral 2 and the remainder of the principal branches originate below 
and in front of it. Thus, the two regions of the tree have a different 
topography with reference to the pulmonary artery, but the latter has 
no fundamental influence on the structure of the two parts, nor does 
