260 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
In the development of the embryonic Melophagus the head-fold 
has a history similar to that of these cephalic segments of Musca. 
As the embryo advances in its development the head-fold diminishes 
in size, and the stomodeum, which first appeared at some distance 
from the anterior end of the egg on the head-fold, migrates forward 
towards its definitive position at the anterior end, probably as the 
result of a longitudinal concentration or shortening of the germ- 
band. The history of the tail-fold is the same. It also becomes 
reduced in size as the proctodeum migrates towards the posterior 
end of the embryo. This migration is much more rapid than that of 
the head-fold. Thus, when the young Melophagus larva is finally 
born, it resembles externally in a marked degree the new-born fly 
larva. It consists of twelve segments, of which eleven belong to the 
body, and one, which represents the rudiment of the head-fold, to 
the head; the head- and tail-folds have disappeared, the germ-band 
having entirely retreated from the dorsal side of the embryo. 
The further development of the cephalic discs goes on hand in 
hand with this migration of the stomodeum towards the anterior 
pole of the egg. The paired discs develop much more rapidly than 
the median one. The latter remains a mere thickening of the ecto¬ 
derm, without showing signs of invagination, during the entire 
migration of the stomodeum, so that when this organ has reached 
its final position at the anterior end of the egg the median disc 
has simply changed its position. It is now an ectodermal thicken¬ 
ing on the ventral side of the embryo just beneath the mouth 
opening (PI. 3 , Fig. 22, and PI. 4 , Fig. 2b, dsc. ce. m.). 
The paired discs, on the contrary, early in the course of their 
movement forward begin to invaginate. The convex margin of 
each of the crescentic thickenings becomes much thicker than the 
concave, and along this outer (convex) margin a groove-like invagi¬ 
nation is formed (PI. 2, Fig. 14, dsc. ce .). At the stage of devel¬ 
opment represented by Figure 14, the right and left halves of 
the germinal band in the region of the body have not proceeded 
more than half way towards the dorsal side of the egg, as will be 
seen in Figure 15 ( tae. ms'drm ), which represents a cross-section 
through the middle of the same embryo as the one shown in Fig¬ 
ure 14. The endoderm has not yet extended to the ventral side of 
the yolk (Figs. 14, 15, en'drm.), the tracheal invaginations form 
deep sacs (Figs. 14, 16, Vvag. tr .), and the nervous system has not 
yet made its appearance. 
