664 General Notes. [July, 
same time the hypoblast cells begin to form the mesenteron, 
men, 
On the tenth day the ventral flexure is formed by a deepening 
of the transverse furrow between the seventh and eighth segments. _ 
As this furrow deepens and the embryo increases in size, the last 
segment grows in length. At the same time the embryo curves 
round toward the ventral surface, the end segment being bent 
round against the head. The eighth segment is longer than the 
others except the head. Even as late as the twelfth day, when 
the nervous system is far developed in other parts of the body, in 
the eighth segment the tissues are imperfectly differentiated, the 
` nerve cords not showing any ganglia, but lying on the epiblast 
and not quite separated from it. Ata later period of development 
the anal segment is constricted off from the eighth, while from 
the anterior part of the latter, the additional segments formed in 
the course of development are derived. These additional seg- 
ments are therefore intercalated between the seventh and ninth. » 
Just before the appearance of the ventral flexure the embryo 
develops a cuticular envelope over the whole surface of the body. 
This is the so-called amnion of Newport. Just before the forma- 
tion of the ventral flexure the nervous system is formed. 
first traces of this consist in a thickening of the epiblast on each 
side of the middle line; this is soon followed by the formation of 
a shallow furrow between the thickened parts; this longitudinal 
furrow corresponds with that described by Metschnikoff in 
Strongylosoma. 
a 
present for each segment except the last. The posterior por- 
tion of the nerve cords is completed at a considerably ene 
= stage of development. The nerve cords are widely separated, bit 
a are connected by a thin median portion. In later embryonic lite 
ae are closely approached to one another, and almost form one 
cord. 
On the eleventh day the embryo has increased considerably a 
size. The ventral flexure is complete, and the animal lies wit 
the long end segment folded closely against the rest of the z $ 
the end ofthe tail being against the stomodæum. The me 
system is now completely separated from the epiblast, an ha 
_ epiblast has assumed the adult form. It now separates 4 seco 
_ membrane like that which is formed on the tenth day. ee 
_ The splanchnic layer of mesoblast covers the mesenteron, ™ 
æ 
2 _Stomodeum and proctodeum be hago he 
_. Within the yolk, which is still present in great qual ae 
_ body-cavity, there are present a number of hypoblast cells. / sys- 
‘as has already been mentioned, give rise to the cir culatory <a 
tem and to various muscles. They may, therefore, benata 
ts 
