ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 35 



the trachea is at certain points suiTOunded by a ring made up of 

 several layers of chitin. 



Wings of the Hymenoptera,* — Dr. W. Breitenbach points out that 

 in an almost completely developed wing of a pupa of a Hymenopterous 

 insect it is possible to see that it is folded in very various ways ; 

 hitherto no connection between these foldings and those of the adult 

 wing have been ever detected. Bearing in mind that the wings are 

 ti'aversed by tracheal branches, which are invested by a thick layer 

 of chitin, and that the whole surface of the wing is covered by small, 

 closely-set hairs, we have further to note that at some points the 

 chitinous layer is almost or altogether completely absent, while the 

 tracheae are reduced and the hairs sparse. Such points may be united 

 by a line, rendered evident enough by the slight development of the 

 hairs thereon. These peculiarities enable us to distinguish the 

 position of the embryonic folds, which were present along these lines ; 

 and the absence of the hairs and the feeble development of the chitin 

 is at once to be ascribed to the mere mechanical fact that along these 

 lines there was not sufficient room for them to be developed. 



Development of the Dorsal Vessel of CMronomns.t — Herr A. 



Jaworowsky deals especially with the musculature of this vessel, and 

 believes that he has succeeded in demonstrating that it, and the valves, 

 are derived from a bilateral rudiment, or, in other words, from two 

 distinct rows of cells. His studies on post-embryonal development 

 lead him to the conclusion that there is a muscular cell for every 

 nucleus found in the contractile wall of the dorsal vessel ; that each 

 circular muscle consists of two lateral cells, which only partly fuse 

 with one another in the middle line ; while the valves themselves are 

 derived from part of the muscle-cells. 



These three conclusions are severally supported by the following 

 observations. (1) In a larva of Ghironomus variegatus one day old, each 

 of the two lateral walls of the dorsal vessel is seen to consist of a series 

 of spindle-shaped elements, with a distinct round nucleus ; the con- 

 tents of the cells are finely granular and but feebly refractive, and it 

 is to this last circumstance that we must ascribe their having been so 

 frequently overlooked ; (2) a series of observations demonstrate that 

 the ends of the spindle-shaped cells elongate in the direction of the 

 middle line, part of each becomes connected with that of its fellow, 

 and between the rest of each cell there extends a tendonous band. The 

 physiological independence of the two sides was spoken to by an ex- 

 periment in which the author found that the final contractions of 

 the cardiac tube took place on one side only, and, during its systole, 

 only the wall of one side was di-awn towards the middle line, (3) 

 The nucleus of the muscle-cell, which is visible during the systolic 

 period, becomes obscured at the diastole ; at the beginning of the 

 systole the contractile vessel turns toward the dorsal wall, and the 

 consequence of this is that the lateral part of the muscle-cell projects 

 into the lumen of the vessel ; so that there is periodically a gi'eater 



* Zool. Anzeig., iii. (1880) pp. .522-3. 



t SB. K. Akad. Wiss. Wien, Ixxx. (1880) pp. 2R8-.'58 (5 pis.). 



D 2 



