Joseph Marshall Flint 7 
head gut into dorsal and ventral portions. From the ventral segment 
arises the respiratory system, while the dorsal is transformed into the 
cesophagus. About the level of the sinus venosus, the lungs appear as an 
unpaired dilatation of the ventral section and, synchronously, the trachea, 
also unpaired, is developed from the head gut just above it. Although 
the two structures appear simultaneously, the anlages, according to 
Uskow, are quite independent. Fol, 84, finds the origin of the lungs in 
a human embryo 5.6 mm. long as lateral diverticule on the head cut just 
behind the series of gill pouches. He is inclined to believe with Gotte 
in the transformation of the last pair of gill pouches which have dis- 
appeared in the phylogeny of vertebrates into the respiratory apparatus. 
His, 87, recognized the anlage of the human lung before the flexion 
of the embryo, that is to say, about the third week. It appears as a 
groove in the ventral part of the anterior segment of the intestine which 
becomes flattened just below the Fundus branchiales into a sagittal 
fissure and divides into an anterior and posterior half. From the former 
the trachea is formed, while the latter develops into the cesophagus. 
The respiratory portion begins above as a groove and ends below at the 
level of the auricles in a widened projection. From the latter, the lungs 
are evolved, while the former yields the trachea. At first, there is no 
medial division of the unpaired anlage which, save through the thickness 
of its epithelial lining, it is difficult to differentiate in the early stages. 
At the end of the first month the separation of the trachea from the 
cesophagus, beginning at the caudal extremity and proceeding upwards, 
is complete. And, as this separation takes place, there is a bilateral 
division of the anlage, which yields the primitive bronchi. These bend 
sharply dorsalwards, like a horseshoe, to embrace the cesophagus. The 
dilated primary lung sacs formed on these divisions are asymmetrical, 
the cause of which is probably to be sought in the first anlage of the 
lungs, which, according to His, does not show bilateral symmetry. 
Up to this time Kélliker, Uskow, and His have agreed in their 
observations that the respiratory apparatus of mammals is derived from 
an unpaired anlage, but Willach, 88, in following the pulmonary system 
- of the mole believes the trachea arises from an unpaired anlage, while the 
lungs originate as paired structures. The asymmetry of the anlage 
according to Willach is probably responsible for the greater development 
of the right over the left lung. In rats and mice, the process of develop- 
ment as described by Robinson, 89, agrees, in general, with the results 
obtained by His, Kélliker, and Uskow. Stoss, 92, and Bonnet, 92, in 
the study of sheep give results which accord with the findings of Uskow 
and Kolliker in rabbits, while Minot, 92, in his account of the evolution 
