230 F. G. HEATHCOTE. 



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 stomodseum. 

 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 



