RESPIKATORY OROAXS IX ARAXE.E. 
45 
mentioned fold, and which overlies the segmental tube, has 
increased considei*ably in thickness in each segment. Its 
cells (»i., fig. 6) are no longer cubical and one-layered, as they 
are in fig\ 5, bnt have become spindle-shaped, elongated 
longitudinally to the body, and arranged in several layers. 
Each such group of elongated cells forms a bundle whose 
ends are in contact with the anterior and posterior basal 
edges of the appendages, thus completely bridging over the 
.segmental tubes, and ultimately gives rise to a corresponding 
segment of one of the two great ventral longitudinal muscles 
of the abdomen. 
The ectodermal areas to which these muscular 
segments are attached are of primary importance 
in enabling us to determine the homologies of the 
tracheae. In their eailier stages it will be seen that these 
areas (nr. 7-11, figs. 6 and 27) are precisely similar to each 
other in position and arrangement with regard to each 
appendage. They occupy the visceral surface of the medial 
half of each post-appendicular (intersegmental) in-folding,^ 
and their transverse extension is at first nearly the same as 
that of the segmental tubes with which the areas alternate. 
Each contact area soon becomes marked by the appearance 
of the intermuscular tendons or en toch on drites. 
These organs, whose mesodermal origin has already been 
demonstrated by Schimkewitsch (’94) are formed in various 
parts of the body by the fusion and metamorphosis of the 
ends of the muscular cells at those places where the ends of 
two or more muscle-bundles come in contact with each other 
and with the hypodermis." In the embryo they form a non- 
' For the area (ar. 7) Ijetween segments 7 ami 8 there is, of course, 
no post-appendicular in-folding, hut its ijosition is otherwise precisely 
similar to that of the othei-s. 
- Schimkewitsch, to whom we mainly owe our knowledge of the 
development of the entochondrites in Arachnida, has a somewhat 
different view of the conditions necessary for the formation of these 
organs. He says ('94, j). 20(5) : “ Mann kann hehaupten, dass in jenen 
Fiillen, wenn zwei Muskelanlagen einander entgegen wachsen, sie mit 
einander verwachsen ; wenn sie aher unter einem Winkel zusammen- 
