192 GEO. S. HUNTINGTON 
II. The mutually interchangeable character of conductory 
and respiratory anlages in the most primitive mammalia. 
III. Variant fluctuations and mutations of the mammalian 
bronchial system. 
TCrsuprayep lor: 
II. The lungs of Aplacentalia develop on lines identical with 
those determining the placental pulmonary unfolding until the 
main features of the future bronchial pattern characteristic of 
the adult are clearly mapped out in the disposition of the pul- 
monary entoderm (5, 32, 35). At ‘birth,’ under the stress of 
altered physiological conditions, they revert for a period to an 
earlier phylogenetic phase. The bronchial pattern is wiped out 
by the sudden distension of the lung and the entire future con- 
ductory apparatus is employed for respiratory exchange. We 
are probably not far from right if we picture the early promam- 
malian lung as presenting about the conditions seen in the third 
ontogenetic stage of the monotreme and marsupial organ, in 
which the immature embryonic lung at the moment of the 
‘birth’ is ‘blown up’ by the first intake of air and reverts tempo- 
rarily to its reptilian prototype. The aplacentalian lung of this 
stage suggests events which might have occurred if a reptilian 
lung began to experiment with adaptations to mammalian or- 
ganization, and the experiment was interrupted. The gradual 
return of the aplacental lung, during the fourth developmental 
period, to the temporarily abandoned type of placental bronchial 
organization points the evolutionary path along which the rep- 
tilian lung attained the mammalian form. The aplacental onto- 
genetic pulmonary cycle demonstrates the developmental poten- 
tiality of the primitive mammalian pulmonary anlage. It 
proves that the early stembronchus and its primary derivatives, 
even after they are grouped into the main lines of the future 
bronchial pattern, are not yet bronchial structures in the sense 
in which we employ the term in adult anatomy, although they 
outline clearly the path along which the subsequent differentia- 
tion of the conductory passages and the respiratory alveoli 
occurs. The early bronchial system of the placental embryo 
still retains the archeal character of a potential respiratory 
