PRATT: IMAGINAL DISCS. 
261 
An embryo which has advanced somewhat beyond the stage 
just discussed is illustrated in Figures 18-21 (PI. 3 ), which repre¬ 
sent sections through a single animal. The stomodeum (Fig. 18, 
stmd.) has become a deep invagination, which is situated on the dor¬ 
sal side of the embryo, 0.16 mm. from its anterior end. It has thus 
moved 0.17 mm. nearer the anterior end than it was at the stage 
represented in Figure 12. The inner end of the invagination has 
now fused with the endoderm. The forward ends of the paired 
cephalic discs appear in this section (Fig. 18, dsc. ce.). At a distance 
of 0.08 mm. back of this section is the section represented by Figure 
19, which shows these discs where the invaginations are deepest. 
The invaginations are here long, curved, deep slits, one on each 
side of the stomodeum, which have a diagonal position in the head¬ 
fold, the median end of each being directed posteriorly. It will 
also be observed that in this embryo the endoderm has completely 
enclosed the yolk. At the anterior end of the embryo it has fused 
with the stomodeum (Fig. 18), while at the posterior end it is seen 
to be continuous with the proctodeum (Fig. 21, en'drm.). The nerv¬ 
ous system has made its apj)earance in the form of a pair of longi¬ 
tudinal cords of cells on the ventral wall of the body, one on each 
side of a mid-ventral ectodermal ridge (PI. 3 , Figs. 18-20, n. v.). 
In the head-fold a longitudinal nerve-cord makes its appearance 
immediately beneath each cephalic invagination and on the median 
side of the slit-like invagination (Figs. 18, 19, gn. cb.) ; these con¬ 
nect with the ventral nerve-cord at the forward end of the embryo. 
These nervous fundaments in the head-fold are undoubtedly the 
beginnings of the cerebral ganglia. This early establishment of a 
relation between the brain of the animal and the paired cephalic 
discs is important, since the relation becomes more intimate as 
development advances. In the larva and pupa the cephalic discs 
are so firmly joined to the cerebral ganglia that Weismann (’ 64 ), 
who first discovered the corresponding discs in Musca, gave them 
the name “ Hirnanhange.” 
A pair of large spherical bodies, apparently of nervous tissue, 
appears in the head-fold at this stage of development (PI. 3 , Fig. 18, 
gn. ala.) at the right and left of the stomodeum, and near the for¬ 
ward ends of the cephalic invaginations. The origin of these very 
noticeable bodies I did not observe ; but they arose between the 
stages represented by Figures 14 and 18, and probably from the 
ectoderm. They are at this stage quite unconnected with any other 
