THE TRACHEAL SYSTEM. 335 



The originally simple tracheal invaginations (Fig. 158 E, tr) first 

 widen at their bases, and then soon give off the tracheal branches 

 as diverticula, while the narrowed aperture of the invagination is 

 retained as the stigmatic branch and stigmatic aperture. The two 

 longitudinal trunks of the tracheal system arise from the consecutive 

 tracheal invaginations; these give rise to horizontally placed out- 

 growths, which grow out longitudinally until they meet and finally 

 fuse with those of the next segment (Butschli, No. 11). Only 

 in late embryonic stages is the cuticular tracheal intima secreted. 

 The tracheae become filled with air, according to "Weismaxn (No. 

 87), to a certain extent even before the embryo hatches, the air 

 being exuded, as it appears, from the tissues and the body-fluid. 



The further development of the tracheal ramifications is brought about, as far 

 as has yet been observed, by the continuous formation of diverticula. The 

 branches which thus arise are therefore intercellular structures. On the other 

 hand, it should be mentioned that the finest tracheal branches are intracellular 

 canals. Although Schaffer (No. 124a)* has correctly pointed out that the 

 difference between these two methods of formation is not of any great impor- 

 tance, since in both cases there is merely an increase of surface (of a cell-plate 

 in intracellular origin, or of a few cells in intercellular formation), the distinc- 

 tion has a certain interest when we compare the condition in Peripatus. The 

 tracheae of Peripatus consist of numerous very fine tubes which, united to form 

 a tuft, arise from a short funnel connected with the stigma. We may perhaps 

 consider the fine tubes of Peripatus as equivalent to the intracellular and the 

 funnel to the intercellular portion of the tracheal system of the Insecta. 



At certain stages of development the tracheae bear a strong resemblance to 

 the rudiments of the salivary glands f and the Malpighian vessels. This circum- 

 stance, as well as the position and number of these invaginations in the 

 Hymenoptera, gave support to the view (held by Butschli, No. 11, Grassi, 

 No. 32, and to some extent also by Carriere, No. 13) that we have in the 

 tracheae and these glands homologous organs. If we consider the anatomy of 

 Peripatus, we shall find that there are objections to this view. The apparently 

 irregular distribution of the tracheae in Peripatus, and the facts that glands 

 (salivary and excretory) similar to those of the Insecta and perhaps homologous 

 with them are also present, show that the agreement in position and in number 

 is of no consequence. Above all, however, the tracheae of Peripatus in structure 

 differ greatly from the glands under consideration. Even Moseley's view that 

 the tracheae are transformed integumental glands, a view also held by Palmen, 

 offers many difficulties. Apart from the circumstance that, in forms that stand 

 nearer to the conjectural racial form of the Tracheata, integumental glands of 

 this kind are not known to exist, the transformation of a secreting organ into 

 an air-filled respiratory organ presupposes a change of function difficult to 

 imagine. It is therefore most probable that we must regard the tracheal 



* Here also the literature on this point will be found. 



t In the efferent portion of many spinning glands also a spiral thread 

 altogether similar to that found in tracheae develops. But the fact that a 

 similar spiral thread occurs also, for instance, in the vas deferens of the 

 Ci/theridac shows that no weight can be laid on this circumstance (KAUFMANN). 



