252 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
only approximately by comparing it with other embryos. I should 
not, therefore, have selected the eggs of Melophagus for the inves¬ 
tigation of this phase of insect development. My problem, as the 
title indicates, was quite a different one, the study of imaginal discs; 
but while searching for embryos containing these structures, I 
collected a considerable number illustrating all the different phases 
of the animal’s development. 
The egg of Melophagus has almost exactly the shape of that of 
Musca, as observed by Weismann (’ 64 ) and Blochmann (’ 87 ). It is 
an elongated cylindrical object, tapering at the poles, being blunter 
at the posterior than the anterior end, with a length of 1.2 mm. 
and an average breadth of 0.3 mm. at its widest part. As seen from 
the side, its ventral outline is convex, its dorsal outline slightly con¬ 
cave. It is covered by a two-layered chorion, the outer layer being 
much thicker than the inner one, and by a delicate vitelline mem¬ 
brane. As it lies in the maternal uterus the main body-axes of the 
developing embryo correspond to those of the mother’s body. The 
micropyle forms a large funnel-shaped depression in the anterior 
end of the chorion (PI. 1, Fig. 4), which sinks deeply into the 
yolk, and the deep dent it makes deforms the anterior end of the 
developing embryo until near the termination of the embryonic 
period. The micropyle is always found filled with a dense mass of 
spermatozoa. 
The very young uterine egg has also a structure similar to that of 
the egg of Musca, but differs from it in one important particular. It 
consists of a web of granular protoplasm within which lies a mass of 
spherical yolk granules, but the peripheral layer of clear protoplasm 
is exceedingly thin (PI. 1, Fig. 1). It has thus no “ Keimhaut- 
blastem,” as Weismann has called the thick peripheral layer of clear 
protoplasm first found by him in the muscine egg. This is rather 
remarkable, as the presence of the Keimhautblastem is characteris¬ 
tic of the higher insects, having been demonstrated in Musca by 
Weismann (’ 64 ) and others, in the Coleoptera by Heider (’ 89 ) and 
Wheeler (’ 89 ), in the Lepidoptera by Bobretzky (’ 78 ), in the 
Hymenoptera by Grassi (’ 84 ), and in the Ilemiptera by Witlaczil 
(’ 84 ), and the absence of it is also characteristic of the lower insects, 
as shown by Ayers (’ 84 ), Heymons (’ 95 ), Wheeler (’ 89 ), and Korot- 
neff (’ 85 ) in the Ortlioptera, and by Brandt (’ 69 ) in the Pseu- 
doneuroptera. The only other instance with which I am acquainted 
of the failure of this layer in one of the higher insects is in Lucilia r 
