28 Jenkinson, Germinal Layers of Vertebrates. 



Several accounts have been given of the process of 

 germ-layer formation in this group ; the best is, perhaps, 

 Wilson's work on the " Embryology of the Sea-Bass " 

 {Serranus). In Serranus the blastoderm consists of a 



z> 



Fig. 13. 

 Serranus (after Wilson). 



A. The blastoderm lying on the nucleated periblast {par.) is cut 

 sagittally. Ingrowth of cells is just beginning at the dorsal lip {d.l.). 



B. Ingrowth of cells at the anterior edge (ventral lip). 



C. The same at one of the lateral lips. 



D. Sagittal section after complete enclosure of the yolk ; k.v. Kupffer's 

 vesicle; y.p. yolk-plug ; 7n.p. medullary plate ; 11. ch. notochord ; end. endo- 

 derm ; other letters as before. 



superficial epidermic layer of flat cells, covering three or 

 four layers of closely packed polyhedral cells below. 

 There is a slight segmentation or subgerminal cavity 

 between it and the yolk. 



At one point — the posterior middle point — of the 

 edge of the blastoderm a thickening, followed by an 



