460 F. G. HEATHOOTE. 
segment, and from its anterior part the future segments 
formed in later life are developed. Just before the first 
appearance of the ventral flexure when the body segments are 
fully formed, the embryo developes a cuticular envelope over 
the whole surface of the body. This may be seen during the 
first formation of the ventral flexure surrounding the body but 
hanging loosely from it. This envelope is the so-called 
amnion of Newport. 
Just before the first trace of the transverse furrow which 
marks the beginning of the ventral flexure has made its 
appearance, the nervous system begins to be formed. The 
first traces of this consist in a thickening of the ectoderm on 
each side of the middle line. This is soon followed by the for- 
mation of a shallow furrow between the thickened parts ; this 
longitudinal furrow corresponds with that described by Mets- 
chnikoff in Strongylosoma. Fig. 31 shows the furrow and the 
ectodermal thickenings. Fig. 32 shows a later stage where the 
nerve-cords are almost separated from the ectoderm. The 
bilobed cerebral hemispheres are formed first and the nerve- 
cords are formed from before backwards, the posterior portion 
not being complete till a considerably later stage of develop- 
ment. 
The nerve-cords are widely separated, but are connected by a 
thin median portion. In later embryonic life they are closely 
approached to one another and almost form one cord. 
On the eleventh day the embryo has increased considerably 
in size. The ventral flexure is complete and the animal lies 
with the long end segment folded closely against the rest of 
the body, the end of the tail being against the stomodzum. 
The nervous system is now completely separated from the ecto- 
derm, and the ectoderm has now assumed its adult appearance. 
It now separates a second membrane like that which I have 
already described as occurring on the tenth day. 
These two membranes I regard as equivalent to two moults 
of the animal. The nerve-cords have considerably altered its 
appearance ; it has sunk deeply into the interior of the body 
except in the end segment and now lie closely beneath the 
