448 TEXT-BOOK OF ENTOMOLOGY 



Weismann did not observe the earliest steps in the process of formation of the 

 stigma and main trunk of the tracheae, which Biitschli afterwards clearly de- 

 scribed and figured. 



Weismann, however, thus describes the mode of development of the intima ; 

 after describing the cells destined to form the peritoneal 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. vii, 97 A, a b 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 homogeneous layer of a tissue, 

 whose origin from cells is recognized only by the regular position of the rounded 

 nuclei (Taf. vii, Fig. 97, B). 



"Then as soon as the wavy bands of the intima entirely disappear, and it 

 forms a straight, cylindrical tube, a fine pale cross-striation becomes noticeable 

 (vii, 97, /?, int), which forms the well-known 'spiral thread,' a structure which, 

 as Leydig has shown, possesses no independence, but arises merely from a par- 

 tial 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 trachese, is confirmed by the embryological de- 

 velopment, 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 corresponding histologically to 

 a cellular membrane. On the contrary, 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, which 

 in the embryo and the young larva of Musca stands in relation to the thickness 

 of the lumen." 



The trachea, then, consists primarily of an epithelial layer, the 

 "peritoneal membrane," or the invaginated epiblast; from this layer 

 an intima is secreted, just as the skin or cuticle is secreted by the 

 hypodermis. We may call the peritoneal membrane the ectotn'li<'<t, 

 the intima or inner layer derived from the ectotrachea the < J H< la- 

 trachea. The so-called " spiral threads " are a thickening of the 

 endotracheal membrane, sometimes arranged in a spiral manner. For 

 these chitinous bands we have proposed the name tcenidia (Greek, lit- 

 tle bands). 



As to the origin of the spiral thread our observations 1 have been 

 made on the caterpillar of a species of Datana, which was placed in 

 alcohol, just before pupation, when the larva was in a semi-pupal 

 condition, and the larval skin could be readily stripped off. At this 

 time the ectotrachea of the larva had undergone histolysis, nothing 



i Amer. Naturalist, May, 1886, p. 438. 



