GENERA L MORPHOL OGY. 355 



granular contents, occasionally intermixed with minute fat 

 granules ' [2, p. 77]. 



' As soon as these systems of cell-strings are capable of 

 isolation, they are found to be no longer solid ; but the larger, 

 at least, exhibit a small lumen into which the convexities of 

 the cells, forming the wall, project. The lumen is filled by a clear 

 fluid and exhibits a special limitation due to a slight thickening 

 of the cell-walls bounding the tube.' 



' This thickening or rather delamination of the boundary- 

 wall forms a structureless intima, which at first follows 

 the outline of the projecting spheroidal cells. As the intima 

 thickens, the cells become fused by their adjacent walls, and 

 the wave-like contour of the intima is straightened ; the 

 vessel then becomes a cylindrical tube and slight transverse 

 striae appear. These become more and more distinct, until 

 at length they are seen as the spiral thread, which, as Leydig 

 has already remarked, is not an independent structure, but a 

 mere partial thickening of the intima [123, p. 387]. Meyer's 

 [149] idea that the spiral thread is produced by a rupture of 

 the intima, due to the entrance of air into the tracheae, must 

 be discarded, as the spiral thread is present long before any 

 air enters the tracheae ' [2, p. 78]. This occurs, as Weismann 

 observed, from two to six hours before the escape of the 

 embryo from the egg, when the longitudinal trunks and their 

 principal branches are filled with air, although the small twigs 

 contain no air until after the escape from the egg [2, p. Si]. 



c. The Morphological Significance of the Tracheal System. 



Gegenbaur held [151] that the earliest animals possessing 

 tracheae were probably aquatic, and resembled the larvae of 

 our present Ephemeridae ; that the primitive form of the 

 tracheal system was probably closed, without spiracles ; he 

 further supposed that the renewal of air was effected in 

 primitive insects, as it is now in many aquatic larvae, by tracheal 

 gills, leaf-like or filamentous appendages projecting from the 

 surface of the body, in which brushes of parallel tracheal 



