PRATT: IM AG INAL DISCS. 
263 
paired cerebral ganglionic fundaments {gn. cb.) beneath the paired 
discs have increased in size. Figure 23 shows their connection with 
the ventral cords. The ganglia allata {gn. ala.) have moved to the 
dorsal side of the embryo, where their anterior ends have fused 
with each other, and occupy a median position (PI. 3 , Fig. 22, and 
PI. 4 , Fig. 26, gn. ala.), while their paired posterior portions lie 
back of the paired discs (Fig. 23, gn. ala.). A still better idea of 
the position of these bodies is given in Figures 43 and 44 (PI. 7 ), 
although these represent sections of a much older embryo, Figure 
44 {gn. ala.) showing the posterior portions of the ganglia, and 
Figure 43 {gn. ala.) their juncture. Their anterior fused portion, 
even in the earlier stage (PI. 3 , Fig. 22, gn. ala.), has much elon¬ 
gated, and extends forward to a mass of mesodermic tissue, dorsal 
to the forward portion of the stomodeum, which is fast developing 
into a group of muscle-fibres {Ing.), whose later history will be 
found to be interesting. In the dorsal wall of the stomodeum is to 
be seen a slight evagination {gn. f., PI. 3 , Fig. 22; PI. 4 , Fig. 26), 
which I take to be the fundament of the ganglion frontale. It is a 
very transitory structure, soon disappearing without leaving a trace 
in the later history of the animal. 
Segmentation has become much more strongly marked in the 
forward portion of the embryo, but has almost disappeared from the 
hinder portion of it. In Figure 22, which represents a somewhat 
oblique sagittal section, we see three deep grooves in the ventral 
ectoderm, which mark the boundaries between head, prothorax, 
mesothorax, and metathorax, respectively. Two following shallow 
grooves mark the boundaries of the metathorax, the first, and the 
second abdominal segments respectively. 
The arrival of the stomodeum at the anterior end has transformed 
that part of the body of the embryo. The mouth is a transverse, 
slit-like opening (PI. 4 , Figs. 25 and 27, stmd.). Its ventral lip 
is the median cephalic disc, which, however, is still only a thick¬ 
ened portion of the ectoderm. Its dorsal lip is a conical struc¬ 
ture, and projects freely forward above it (PI. 3 , Fig. 22; PI. 4 , 
Fig. 27, big.). This structure is of great importance in the lar¬ 
val life of the insect, for it acts as a sucking tongue by means of 
which the animal ingests its milk-like food. (For a description 
of it in the larva, see Pratt, ’ 93 .) When it first appears, this organ, 
as will be seen in Figure 22, forms the dorsal wall of the anterior 
end of the stomodeum, and its anterior tip projects beyond the 
