PRATT: IMAGINAL DISCS. 
269 
the anus and rectum (PI. 2, Fig. 48, dsc. gen. p .), and the latter 
immediately in front of it. 
«/ 
These two pairs of discs do not make their appearance simul¬ 
taneously, the larger, posterior pair being the first to appear. These 
have the form of two invaginations, one on each side of the anus 
(Fig. 48), which project forward and fuse on the ventral (an¬ 
terior) side of the end-intestine, forming a single flattened sac, 
which partly encircles it (Fig. 49, dsc. gen. p>-) . At its anterior 
end this sac separates into two parts, being in this region again 
paired (Fig. 50, dsc. gn. p.). It is in the outer ectoderm at this point 
that the anterior discs finally make their appearance in the form 
of a pair of ectodermal thickenings (Fig. 50, dsc. gn. a.). These 
do not, however, develop further in the embryo; but in the larva 
they assume a sac-like form and are detached from the ectoderm. 
A description of the development of these discs in the larva has 
already been published (Pratt, ’93). 
4. The Discs of the Internal Organs. — The imaginal discs of 
the internal organs and of the abdominal hypodermis do not appear 
during the embryonic life of the insect. 
Methods. 
The greater part of this investigation was carried on in the 
Zoological Laboratory of Harvard University under the direction of 
Prof. E. L. Mark. It was completed at Haverford College. Most 
of the material was obtained in Germany, but a portion of it came 
from the vicinity of Cambridge, Mass., and of Haverford, Pa. 
The eggs were obtained by dissection from the maternal uterus, 
in which they develop. It was found to be impossible to remove 
them in the fresh condition without injury, on account of the 
extreme delicacy of the chorion. Consequently, in each case the 
mother insect was killed by decapitation; the abdomen was slit 
open and then plunged into a warm corrosive-sublimate solution. 
This fixed the egg, and at the same time hardened it so that it 
could be removed from the uterus. Most of the staining was done 
with borax carmine, which was found sufficient for all ordinary 
purposes, but Mayer’s acid carmine and Ehrlich’s liaematoxylin 
also were used. 
