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Beekeeping 
Later embryonic development. 
In a later stage (Fig. 51, c), the embryonic band on the 
ventral side of the egg has widened and in the next stage 
here illustrated (Fig. 51, d) the band completely envelops 
the egg. In the stage shown in Fig. 51, c the mouth (Mth) 
and anus (An) have appeared as pits. These continue to 
grow into the egg and ultimately join with certain cells on the 
interior to complete the alimentary canal. The portions 
formed by the two invaginations from the outside are the 
fore- and hind-intestine, while the part arising from the 
interior is the mid-intestine. The Malpighian tubes (MT), 
the excretory organs, arise as outgrowths from the anterior 
end of the hind-intestine. The pits ( Sp) which are the rudi¬ 
ments of the spiracles, deepen and send branches forward, 
backward and downward to meet corresponding outgrowths 
from other pits, finally forming the tracheal trunks with 
their commissures and branches. The silk glands ( SlkGl ), 
which function only in the larva, project backward as long 
tubes. 
Segmentation. 
The most striking feature of the late embryo is the fact 
that it is constricted into a series of segments (metameres 
or somites) which are plainly recognized in the larva. These 
segments are characteristic of all insects and part of the 
metameres of the abdomen are still plainly marked off in 
the adult. From the fact that segmentation is recognizable 
in various parts of adult insects and is present in insect 
embryos, it is assumed that this form is characteristic of the 
primitive organism from which all insects have arisen. The 
typical appendages are arranged in pairs on the segments 
but in their later development these appendages are modified 
according to their fate. The stigmata and the ganglia of 
the nervous system are also arranged segmentally at first, 
but this primitive arrangement is later partially lost. The 
segmentation of various species studied does not wholly 
agree, but it is usually assumed that the first six or seven 
