January 27, 1893.] 



SCIENCE. 



45 



section of these tsenidia, would present a superb and convincing 

 picture. 



Not only do tlie teenidia of this special insect (Zaitha fluminea) 

 clearly reveal the fact that they are longitudinally fissured tubules, 

 but these tracheae as clearly show that the so-called spirals are 

 inwardly directed folds of the membrane. This is especially con- 

 spicuous near the spiracles, where the tracheae, both on their inter- 

 nal and external surfaces, exhibit evidences of the fact. Near the 

 spiracles the tracheal membrane is externally studded with minute 

 papillse, which become fewer and smaller as the distance from the 

 spiracle increases. Here the intima is thrown into folds so shallow 

 and so broad that they are often mere grooves, and with nospecially 



to me, as I had observed on Mr. Dienelt's preparation certain 

 structural points which together form the subject of this paper. 



The tracheae of the insect are large, and, as the tanidia are also 

 comparatively broad, the entire collection of tracheal tubes, es- 

 pecially in the principal trunks, readily lend themselves to inves- 

 tigation. It is here an easy task to demonstrate that the taenidia 

 are fissured tubules formed within and from chitinized folds of 

 the intima, the convexity of the folds looking toward the lumen 

 of the trachese, the fissure, as Professor Macloskie has observed, 

 being directed away from that lumen. In balsam mounts, and 

 perhaps somewhat more distinctly in glycerine preparations, 

 under a wide angled, homogenous immersion ^^^-inch objective, 

 the irregular edges of the longitudinal fissure in each tsenidium 

 of the larger tracheae can be seen and studied at the microscopist's 

 pleasure; indeed, so well marked are they that they may easily be 

 seen with Zeiss's apochromatic 6-millimeter objective. 0.95 N. A., 

 and, in favorable circumstances, with Gundlach's dry ^, N. A. 

 0.92. The appearances are in no way those of the diffraction 

 phenomena produced by solid fibres, but rather an aspect which 

 suggests the illumination of a hollow tube by reflection from its 

 walls. The method of focussing which gives this bright band 

 yields a picture of the edges of the fissures, with a more or less 

 brilliant space between them. But by using the method employed 

 by every well-informed microscopist when studying the secondary 

 structure of the diatoms, a different appearance is obtained. The 

 method is nothing more than a certain manner of focussing the 

 objective, but one which produces the "black dot resolution" 

 which has revealed so much of importance in reference to the 

 intimate structure of those silicious plants. The " black dot " 

 focus is as correct when applied to these minute tsenidia as when 

 obtained over the secondary membranes of the diatoms. With it 

 the margins of the tsenidial fissures are separated by a black space 

 that defines them and every irregularity of the edges with con- 

 vincing distinctness. 



These margins are not parallel, a fact which alone would pre- 

 clude the possibility that the appearances are diffraction phe- 

 nomena. Neither is the fissure that separates them even in width. 

 It is narrowed in indescribable ways by the approach of one 

 margin toward the other, by a retreat from each other, and by 

 wavy and more or less crenuiated outlines. In some tsenidia 

 these margins have come in contact and have apparently been 

 united, showing, in many instances, the point of union as an ex- 

 ceedingly slender line, whilst in others the juncture has been 

 obliterated, and in still others the union has been accomplished in 

 such a way that minute and irregular spaces have been left in the 

 course of the original fissure, like little islands of darkness, or of 

 brightness, according as the microscopist uses the black dot reso- 

 lution, or focusses for the illuminated band. 



In Fig. 1 are shown portions of several taenidia exhibiting the 

 fissure, and, although the drawings were originally made with 

 the camera lucida, they are necessarily somewhat diagrammatic. 

 The one two-thousandth inch scale appended, is magnified to the 

 same degree as are the tajnidia, but is applicable to these par- 

 ticular drawings only, and not to the spares between them. In 

 the fii'st two bands on the left-band side the fissure is shown 

 black, as I think it should be, whilst in the others it is left white, 

 to exhibit rather more distinctly for this purpose the irregular 

 margins and the irregular widths of the fissure. In the last two 

 portions on the right hand side, and in the five of Fig. 2, are de- 

 lineated some of the various aspects assumed by the union of the 

 edges of the fissures, and the formation of what may be called 



apertures in a chitinous bridge conspicuous deposit of chitin. Here too the margins of these 



The fissures are so distinctly defined in ordinary preparations shallo%v and groove-like infoldings are crenuiated by the papillje, 

 that sections of the taenidia are not necessary to show them. Such which become more conspicuous as they are presented in profile 

 sections, however, are desirable, but to make them is an impossi- on the edge of the furrow. An attempt has been made to show 

 bility for me. The microscopist that nowadays tries to work this appearance in Fig. 3, where are delineated three broad and 

 alone and without a laboratory at his disposal, and without the incomplete taenidia, with the tapering termination, or the begin- 

 refinements of microtomy and of photo-micrography, can do but ning, of another. Here they are only broad grooves, with no 

 little at a disadvantage. Yet when a taenidium is traced to the appearance of the narrow fissure of the completed taenidium, as 



folded and flattened margin of a large trachea, in some instances 

 the narrow, externally concave fissure can be plainly seen, 

 although confusing diffraction effects must there be contended 

 with. A longitudinal section of the tube, that is, a transverse 



it is now all fissure; in many instances these shallow depressions 

 are even more irregular than shown in the figure. Near the 

 spiracles they are sometimes hardly more than a collection of 

 wrinkles in the crumpled membrane, as is delineated in Fig. 4, 



