330 CHORD AT A. 



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 netireiiteric 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 mesoblnstic 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 embryo 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 abdominal wall of 

 the embryo. Over its surface there ramify vitelline 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 protovertebrcE lying on either side of the noto- 

 chord and a ventral portion which forms the lateral plate. A split 

 occurs between the cells of the lateral plate and forms the coelom, 

 which is thus schizocoelic. 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 coelom) which is continuous through the stalk into the 

 embryonic coelom. 



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 protovertebrae give rise to the vertebral 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 



