54 
W. F. FUKCELF. 
space (ar. 9) occupying the medial region of its visceral 
surface and enclosed by the dotted lines in the figure is the 
area with which the ends of the longitudinal muscles are in 
contact, and to the deepest part of which the entochondrite 
becomes attached. The medial area (ar. 9) of the tracheal 
sac is, therefore, serially homologous with the corresponding- 
area (nr. 8) behind the pulmonary appendage (see p. 20), 
and has consequently nothing to do with the region in which 
the earlier pulmonary furrows appear, nor with any portion 
of the lung-books. It will be observed that owing to the 
presence of the lung-leaves the area (ar. 8) in the pulmonary 
segment is more widely separated from the segmental tube 
{seg. t. 8) than is the case in the tracheal segment. 
The lateral region of the ti’acheal in-folding is of especial 
interest, as it is the only part which is serially homologous 
with the pulmonary sac. It will be remembered that the 
pulmonary sac proliferates in a lateral direction (position as 
in fig. 1), later in a dorsal direction (position as in fig. 3), in 
the form of a hollow tuber-like process creeping along the 
inner surface of the outer epithelium; and that this sac and 
its proliferations yield the cell-material for the formation of 
the fourth and following pulmonary saccules. 
Now the tracheal post-appendicular in-folding begins to 
proliferate laterally simultaneously with the pulmonary sac 
in precisely the same manner and direction. But the walls 
of the tracheal sac have not to furnish cell material for lung- 
saccules, of which no traces are present at any time, and, no 
doubt, on this account the pulmonary sac rapidly outgrows 
the corresponding tracheal sac, and in the stage of fig. 27 
already greatly exceeds it in size. In this figure the groove 
{tr. 1.) behind the tracheal appendage extends dorsally up to 
section No. 16, while the proliferation extends through five 
more sections; and the groove [imlm. 1.) behind the pulmonary 
appendage reaches to section No. 24, while the corresponding 
proliferation extends further likewise through five more 
sections. 
Figs. 35-35a and lOn-lbE represent longitudinal sections 
