156 
ME TAMORPHOSIS. 
with the tracheae, the remainder comes later. The 
former has a pit just behind the procephalic promi- 
nence, and in the posterior region of the dorsal surface 
two pair of small pits are found, which give rise to 
the four malpighian tubes. The rectum is at first a 
simple depression, which extends backwards, and forms 
a canal. Just behind the point where the second maxillae 
will be formed, two fossae appear (Fig. 57, c, //, /), 
which are directed backwards, and these are the silk 
glands. Two other pair of organs are formed as in- 
vaginations of the ectoderm, just in front of the 
mandibles (c, ^), and on the third day the first pair 
disappear. The yolk begins to disappear in parts, 
and becomes concentrated round the future stomach, 
which then forms a cavity. The dorsal vessel is 
formed along the line where the two layers of the 
mesoderm meet dorsally. Later, and a little before 
the larva leaves the egg, the tube commences to 
enlarge, and contains blood corpuscles. Two solid 
cords of cells, unconnected with each other, extend 
from the fourth to the eighth abdominal segments, 
and give rise to the genital organs ; at a later period 
they, however, precede the development of the muscles. 
The stomach is lined with ectoderm, the sides bend- 
ing round and meeting on the ventral surface to form 
a tube. The procephalic lobe bends downwards 
towards the third day, so as to form the upper lip 
(Fig. 57, c, /). The antennae, which grow nearly to 
the third day, appear simultaneously with the buccal 
appendages. Of these there are four pair, of which 
the first pair totally disappear, while the rest form the 
