Joseph Marshall Flint 29 
precocious as I possess later stages where the two systems are in open 
communication at a lower level than is shown in this specimen. At the 
level where the Mesocardium posterior begins (Fig. 7 VM), the epithe- 
lium lining the fore gut is columnar and consists, except in the ventral 
and dorsal angles, usually of a double layer of cells. In the anlage of the 
lungs (Fig. 8), it is slightly higher and shows a more active karyoki- 
netic process. A similar layer of endoblast extends out into the primitive 
bronchi. At the tips, cell division is proceeding rapidly. 'The mesoderm 
of the lungs remains, so far as its differentiation is concerned, practi- 
cally unchanged, but the lateral extension of the left bronchus now 
makes the projection into the ccelom at this level more marked than on 
the right side as the right bronchus, lying in a caudo-lateral direction 
nearer the median plane, does not carry the mesoderm quite so far into 
the right coelomic cavity. On both sides, the Recessus pleuroperitonealis 
Tix HGS 9) 
Text Fic. 9. Section of an embryo 4.5 mm. long, through the lower part 
of pulmonary anlage. O=(Mophagus. C=Celom. BD=—Right stem 
bronchus. RD=—Right Recessus pleuroperitonealis. RS—=Left Recessus 
pleuroperitonealis. VM—Mesocardium posterior. 
may be seen. It is larger and extends higher on the right than on the 
left (Fig. 9 RD,RS). In Fig. 9, the beginning of the formation of the 
dorsal mesentery at the lower level of the lungs is apparent. 
By the reconstruction process, the changes which have been occurring 
in the two preceding stages are demonstrated beautifully in a pig 5 mm. 
long where they are also considerably accentuated. Above (Pl. I, Figs. 
3, 4) is seen a segment of the last gill pouch, while below it, the head gut 
narrows rapidly to a sagittal fissure forming the ventral respiratory and 
the dorsal digestive portion. The pulmonary groove, still in open com- 
munication with the cesophagus, terminates below in the asymmetrical 
