1886.] so-called “Spiral Thread” of Trachee. 439 
of the intima; after describing the cells destined to form the peri- 
toneal membrane, he says: “The lumen is filled with a clear 
fluid and already shows a definite border in a slight thickening of 
the cell-wall next to it. 
“Very soon this thickening forms a thin structureless intima, 
which passes as a delicate double line along the cells, and shows 
its dependence on the cells by a sort of adherence to the rounded 
sides of the cells (Taf. vu, 97 A, æ ó c). Throughout the mass, 
as the intima thickens, the cells lose their independence, their 
walls pressing together and coalescing, and soon the considerably 
enlarged hollow cylinder of the intima is surrounded by a homo- 
geneous layer of a tissue, whose origin from cells is recognized 
only by the regular position of the rounded nuclei (Taf. vit, fig. 
97 B). 
“Then as soon as the wavy bands of the intima entirely dis- 
appear and it forms a straight cylindrical tube, a fine pale cross 
striation becomes noticeable (vir, 97, B, int), which forms the 
Well-known ‘ spiral thread,’ a structure which, as Leydig has 
shown, possesses no independence, but arises merely from a partial 
thickening of the originally homogeneous intima. 
“ Meyer’s idea that the spiral threads are fissures in the intima 
Produced by the entrance of air is disproved by the fact that the 
spiral threads are present long before the air enters. Hence the 
correctness of Leydig’s view, based on the histological structure 
_ Of the trachee, is confirmed by the embryological development, 
and the old idea of three membranes, which both Meyer and 
Milne-Edwards maintain, must be given up.” 
Weismann also contends that the elastic membrane bearing the 
spiral thread” is in no sense a primary membrane, not corre- 
sponding histologically to a cellular membrane. On the con- 
trary, the “ peritoneal membrane comprises the primary element 
of the trachea; it is nowhere absent, but envelops the smallest — 
branches as well as the largest trunks, only varying in thickness, 
u 
which in the embryo and the young larva of Musca stands in 
relation to the thickness of the lumen.” ~ 
„ Lhe trachea, then, consists primarily of an epithelial layer, the 
_ Peritoneal membrane” or the invaginated epiblast ; from this layer ` 
= intima is secreted, just as the skin or cuticle is secreted by the 
“ypodermis, We may call the peritoneal membrane the ecto- 
~ Wachea, the intima or inner layer derived from the 
