ME TAMORPHOSIS, 
155 
meet, while the embryo is becoming shorter as the 
ventral plate curves more and more to the dorsal 
surface. 
The nervous system is developed rather late, and 
appears about the same time as the spiracles. The 
cerebral ganglia are formed as two thickenings of the 
ectoderm, appear about the same time as the antennae, 
and do not become united until late in the develop- 
ment of the embryo. The ventral chain arises as two 
longitudinal swellings, which are at first quite separate, 
the ectoderm being also concerned in its formation. 
From this at a later stage also proceed the transverse 
commissures. The ganglionic chain of the thorax and 
abdomen consists of thirteen ganglia, and is prolonged 
into the head, where three ganglia appear, but are dis- 
tinctly separated from each other. 
Of the tracheal system, ten pairs of spiracles are 
the first to appear before the limbs show themselves, 
and after the amnion is a complete sac. The anterior 
ones appear first, and the invaginated ectoderm, which 
gives rise to the spiracle, grows inwards, and divides 
into an anterior and a posterior branch. Each ring, 
which is now visible, has one pair, except the first ring 
of the thorax and the two last abdominal ones, in 
which the spiracles are absent. The lateral tracheal 
trunk is united to its fellow on the other side, above 
the oesophagus, and also above the rectum. The 
spiral filaments (page 52) appear later. The tracheae 
are filled with liquid, and air only enters them when 
the larva emerges from the egg. 
Part of the alimentary tract, which is blind, appears 
