TUB DEVELOPMENT OF VERTBBRATA. 



223 



terms are needed to describe it : the outer layer of cells is 

 called the epiblast; the inner layer, primitive hypoblast; the 

 central cavity is the archenteron, or primitive alimentary 

 canal ; its opening is the blastopore. 



9. First Vertebrate Features. The gastrula of Am- 

 phioxns is never quite spherical in fig. 1 12, c, it is seen that 

 one side is flattened this is the future dorsal surface. The 

 blastopore, originally marking the future posterior end, soon 

 turns towards the dorsal side. This dorsal position of the 

 blastopore is very characteristic of Vertebrates. If now 

 we examine a transverse section of the embryo (fig. 113, A), 



Fig. 113. TRANSVERSE SECTIONS OF EARLY LARVA o AMPHIOXUB. 

 (From Willey, after Hatschek.) 



In A, the outer Layer is epiblast (with neural plate just differentiated on the 

 dorsal side); the inner, primitive hypoblast. In B, the neural folds are 

 seen ; under the neural plate, the first sign of the notochord is seen, and 

 on either side of it a myococlomic pouch. In C, the neural plate has 

 begun to roll up, the neural folds have closed over it, and the myocoelomic 

 pouches are more distinct. 



we see that in the dorsal flattened region a plate of epiblast- 

 cells is beginning to differentiate from the rest : this is the 

 neural plate, another Vertebrate feature. Soon the gastrula 

 begins to become more elongated along its antero-posterior 

 axis. A cilium is developed from every epiblast cell (fig. 

 112, c), the follicle is broken through, and the gastrula 

 escapes to swim about near the surface of the sea by the 

 constant motion of its cilia. Hitherto its increase in size 

 has been due simply to change from, a solid sphere to a 

 hollow body : there has really been a diminution in material, 

 for all cell-division is work and involves some katabolism, 

 and there has only been the small quantity of yolk to serve 

 as food to the protoplasm. From now onwards the larva 



