24 Vertebrate Embryology 



until the yolk-plug entirely disappears from 

 the surface, and the blastopore is reduced to a 

 narrow slit. The layer of black cells, which 

 now completely surround the egg or embryo, 

 is the upper germ-layer or ectoblast (Fig. 9). 



Carefully prepared sections through the 

 embryo at the time of the appearance of the 

 dorsal lip of the blastopore may show, in 

 the region where the white and black cells 

 meet, a more or less clearly defined zone of 

 cells extending equatorially around the em- 

 bryo. This band is several cells deep, the 

 inner cells passing insensibly into the yolk- 

 cells, the peripheral cells being indistinguish- 

 able from the ectoblast. 



" This ring of cells, as subsequent develop- 

 ment shows, is the beginning of the embryo, 

 and the ring itself is composed of the material 

 which subsequently forms the central nervous 

 system, the mesoderm, the notochord, and a 

 part of the entoderm." ' 



By a process of concrescence, which is closely 

 related to the closure of the blastopore, de- 

 scribed above, this band of cells shifts towards 

 one side of the embryo, and its right and left 

 halves fuse to form a broad meridional band 



' Morgan. 



