104 The Development of the Lungs 
The process is essentially the same whether it occurs either in the neigh- 
borhood of the terminal bud, higher up on the stem, or on the trachea. 
In general, we may say that the lateral and medial bronchi are produced 
nearer the terminal end of the main bronchus, while the dorsal and 
ventral elements are produced somewhat higher up from the stem, often 
where the latter has regained its cylindrical form. . 
If Narath’s interpretation of the bud as reaching up to the last appar- 
ent lateral branch is allowed to stand, then all of the branches except the 
tracheal bronchus must be considered in the sense of Narath as lateral 
productions of the end bud. Narath’s distinction, however, does not 
seem to be well made for, in the pig’s lung at least between the last 
lateral branch and the tip of the stem bronchus, there is always a con- 
siderable portion of the main stem which has a definite cylindrical form 
and terminates in a distinct dilatation at the end. Much as Narath’s 
view would tend to simplify the question, there is little justification, 
therefore, in looking upon the entire distal part of the stem bronchus as 
the terminal bud. On the other hand, there is no essential difference in 
an evagination taking place at ihe bud and in one taking place on the 
stem. 
It may be well to notice certain differences in the behavior of the stem 
at different periods in the life of the organism as well as differences be- 
tween different species. For example, in the pig, the stems seem rela- 
tively more irregular and dilated in size in embryos between 10 and 13 
mm. long, but on the whole are fairly cylindrical throughout the growing 
period. On the other hand, in some species the stems, particularly at 
the growing ends, are quite irregular in shape and may be considerably 
dilated, suggesting somewhat pictures corresponding to the growing lungs 
of reptiles. 
After the formation of the chief branches has occurred, the primitive 
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 by the limited environ- 
ment of the bronchi, it becomes unequal, one fork growing on so rapidly 
to become the stem, that the other is left either as a small bud or a small 
side branch, which develops further when the space relations permit. 
