PRATT: IMAGINAL DISCS. 
257 
The proctodeal invagination ( prd .) has become much deeper, and 
at its inner end the beginnings of the Malpighian tubules have 
made their appearance ( tb. mpg .). The stomodeal invagination 
( strnd .) forms a deep depression in the head-fold a third of a milli¬ 
meter from the anterior end of the egg. A surface view of the 
dorsal aspect of an egg in this stage of development is represented 
in Figure 11 (PI. 2), and one of the lateral aspect, in Figure 13. 
In both these figures the head-fold (pli. ce.) is seen to be sharply 
marked off from the remainder of the dorsal integument by the 
presence of the rudiment of the amnion (am). The tail-fold is not 
marked off with the same distinctness for two reasons : the posterior 
rudiment of the amnion, which appeared in Figure 9, has now dis¬ 
appeared ; moreover the fold itself does not occupy as much of the 
dorsal surface of the egg as it did in the previous stages, as is shown 
by the position of the proctodeum nearer the posterior pole of the 
egg- 
The primitive mesoderm also shows a marked advance in develop¬ 
ment in eggs of this ager. The band or ridge of mesoderm which 
took its origin in the mid-ventral line of the germinal plate has sep¬ 
arated into two lateral bands which, as in other insects, occupy 
positions to the right and left of the mid-ventral line. Figure 13 
shows these bands (tae. ms’drm) as they appear in a lateral view of 
the embryo ; Figure 15 (tae. ms'drm ), in a cross-section. It will be 
seen that the mesodermal cells have entirely abandoned the mid- 
ventral portion of the egg, a space being left between the yolk and 
the ectoderm which represents the fundament of the coelom (coel.). 
The formation of these lateral mesodermal bands, as just described, 
takes place, however, only in the middle portions of the egg. At 
the two ends of the egg and in the head- and tail-folds the meso¬ 
derm does not entirely abandon the median portions of the egg, 
although it expands into the lateral portions of it. The mesoderm 
of the ventral side is continuous at the ends of the egg with that of 
the head- and tail-folds. Thus at each end of the egg there is a 
continuous layer of mesoderm lining the entire inner surface of the 
ectoderm (Fig. 12), instead of two lateral bands, such as are found 
in the middle portions of the egg. 
In embryos of this stage segmentation is distinctly indicated in 
the ventral portions of the germinal plate, ten or eleven segments 
being represented (PI. 2, Figs. 12, 13). The tracheal invagina¬ 
tions also appear (Fig. 13), there being eleven pairs of them, of 
