330 CHORDATA. 
The folds meet in the middle of the embryo and anteriorly, but are 
open posteriorly till the blastopore is nearly closed; then they meet behind 
it and so produce a neurenteric canal. The anterior end of the nerve 
tube swells to become the brain and the eyes and parts of the brain 
arise as described in the general account for vertebrates (see page 
406). 
Immediately below this nerve tube the hypoblast cells in the middle 
line become differentiated into a notochord, and laterally the hypoblast 
also becomes differentiated into a pair of cell-plates which form the 
mesoblast. The topographical relationships of the neural tube, the 
notochord and mesoblastic plates are therefore much the same as in 
Amphioxus, but the last two arise as solid masses of cells, not as hollow 
outgrowths. At the embryonic rim the nerve tube, notochord and 
mesoblast are all merged into a growing mass of cells. 
The embryo then becomes folded off from the rest of the blastoderm 
until it is only connected therewith by a small stalk called the yolk- 
sac stalk, The whole developing organism is then clearly defined into 
the emdryo and its yolk-sac, attached to each other by a short stalk. 
The wall of the yolk-sac and the embryo are alike produced from the 
blastoderm, and we may make matters clearer at once by explaining 
that the yolk-sac is really a huge enlargement of the abdoininal wall of 
the embryo. Over its surface there ramify vzted/ine arteries and veins 
which serve to absorb nourishment for the embryo. 
The mesoblastic plates now grow round ventrally inside the epiblast 
to enclose the yolk-mass. They divide into a dorsal portion which 
splits up into a series of protovertebre lying on either side of the noto- 
chord and a ventral portion which forms the /ateral plate. A split 
occurs between the cells of the lateral plate and forms the ccelom, 
which is thus schizoccelic. This split extends completely round the 
yolk-mass, dividing the mesoblast of the lateral plate into an outer 
somatic layer under the epiblast and an inner splanchnic layer resting 
on the hypoblast and yolk-mass. 
Thus the extra-embryonic part, which we called the yolk-sac, now 
consists of an outer layer of epiblast and mesoblast which we may term 
the serosa (or serous membrane) and an inner layer of mesoblast and 
hypoblast enveloping the yolk and called the yolk-sac proper. These 
two embryonic (or foetal) membranes are separated by a cavity (the 
extra-embryonic ccelom) which is continuous through the stalk into the 
embryonic ccelom. 
It is evident that the serosa is merely the much distended body- 
wall and the yolk-sac proper a similarly distended part of the gut-wall. 
The protovertebre give rise to the vertebra] column and myomere 
muscles, 
The gill-slits appear at the side of the neck, and from them there 
soon protrude a number of long, delicate gill-filaments, the external 
gills which are lost before hatching, their bases alone persisting as 
the permanent gills. 
The organs in general arise much as narrated in the general vertebrate 
account (see pages 405-430). 
In comparing this development with that of Amphioxus much assist- 
ance will be rendered by study of the frog, which in the amount of yolk 
