PRATT: IMAGINAL DISCS. 
2(35 
cerebral ganglia, with which they lie in contact. The paired 
discs have thus become a single structure with a single, median, 
dorsal, anterior opening, backward from which extends an irregu¬ 
larly Y-shaped invagination. Figure 30 (PI. 5 ), which represents 
a sagittal section, shows the median opening and the median por¬ 
tion of the invagination. Figure 31, which represents a parasag¬ 
ittal section, shows one of the branches of the Y-shaped invagina¬ 
tion ( else. ce.) ; and Figure 32, a parasagittal section laterad of the 
one last mentioned, shows the irregular structure of the disc. 
The ventral median disc (Fig. 30, else. ee. m.) has not changed 
its position or character, except to become much thicker. 
A comparison of the structures of the embryo represented in 
Figure 22 (PI. 3 ) with those of the one represented in Figure 29 
(PI. 4 ) , which is the same as that of the last three figures dis¬ 
cussed, shows that the development in the latter has been consid¬ 
erable. The muscle fibres of the sucking tongue are distinctly 
developed, but the nerve which proceeds from its base to the ganglia 
allata, and, also, these ganglia themselves, have not changed their 
character. The paired ventral nerve-cords and the cerebral gan¬ 
glionic fundaments (Fig. 31) have developed considerably, the 
former having fused with the median ectodermic ridge, which has 
now effected a separation from the ectoderm, and thus come to 
form with the paired nerves a single structure. Neuroblasts are 
present throughout the entire extent of both ventral and cerebral 
nerve-masses, but are not present in the ganglia allata nor in the 
median nerve proceeding from it. It will also be noticed that the 
anus has shifted its position from the hinder end of the animal 
to the ventral side near the hinder end of the mid-intestine, the 
position it occupies in the larva. A communication has also 
appeared between the stomodeum and the intestine (Fig. 30). 
The involution of the head of the embryo now takes place. An 
ectodermic fold starts back of the cephalic discs, both dorsally and 
ventrally (PI. 5 , Fig. 30), and grows rapidly forward towards and 
over the mouth. The mouth, together with the ventral disc ( dsc. 
ce. m.) just below it, the muscular tongue ( big .), and the common 
opening of the dorsal discs (of. m.) just above it, is rolled in by 
this process. A new mouth is thus formed (PI. 6, Fig. 34, or.), 
and back of it a new portion of the digestive tract ( phy .), the so- 
called pharynx of Weismann and Van Pees, described by them in 
the muscidian larva. 
