EESPIRATORY ORGANS IN ARANE.p]. 
23 
witli the corresponding section, fig. 17, of a nincli later 
stage.) 
Formation of the pulmonary saccules. — Next to their position 
on the posterior side of the appendage, the precise manner in 
which the saccules begin to form is the point of greatest 
interest and importance, when considered with regard to their 
possible direct origin from gill-lamellm. 1 have been 
successful in obtaining a number of excellent sections through 
the region in question, showing the cell-boundaries with 
perfect distinctness. The position of these and of the nuclei 
of each individual cell have been drawn with the aid of a 
drawing apparatus in the sections figured in the plates, which 
are in this respect exact reproductions of the original sections 
(see p. 8). 
The three figures 7, 0, and 8c represent longitudinal sections, 
cut parallel to the axis of the appendage, and, as nearly as 
possible, through the same region of the latter, in each case 
indicated by the line marked {fig. 8c) in fig. 8. All these 
sections are through the region in which the first fuiTOW 
appears, and represent three consecutive phases following 
close upon one another. 
In the youngest stage (fig. 7) no trace of the furrow is 
apparent, and the appendicular epithelium is composed entirely 
of elongated cylindrical cells. 
In the next stage (fig. 9), however, the distal wall of the 
oldest pulmonary saccule has appeared, and is seen still better 
developed in fig. 8c. The formation takes place as follows : 
A cleft {cl. 1) in the epithelium appears on its internal surface 
at the junction of the posterior and distal sides of the 
a])pendage, while a similar cleft (the first pulmonary furrow, 
/. 1) is formed almost simultaneously on the outer surface. 
'I’he cylindrical cells between these two clefts immediately 
begin to shorten to about one-half of their former length and 
rearrange themselves as a one-layered epithelium, whose basal 
and free surfaces are now represented by the internal and 
external clefts respectively. 
'I’he proximal surface of the first pulmonary' furrow is still 
