GENERAL MORPHOLOGY. 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 [128, 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 trachez, must 
be discarded, as the spiral thread is present long before any 
air enters the trachez ’ [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. 81]. 
ce. The Morphological Significance of the Tracheal System. 
Gegenbaur held [151] that the earliest animals possessing 
tracheze were probably aquatic, and resembled the larve of 
our present Ephemeride; 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 larvee, by tracheal 
gills, leaf-like or filamentous appendages projecting from the 
surface of the body, in which brushes of parallel tracheal 
