lo ]y.^¥:.IN'S>0^, Genni7ial Layers of Vertebrates. 



groove are disposed radially about it ; this arrangement 

 marks the beginning of a process of overgrowth and 

 ingrowth which becomes more obvious as development pro- 

 ceeds {Fig. 2, B, C, D). It is then seen that a fold of small 

 cells has grown over a certain area of yolk-cells. This 

 fold consists naturally of two sheets, an outer and an 

 inner. The cells of the outer sheet resemble closely 

 the small pigmented cells of the animal hemisphere into 

 which they are uninterruptedly continued ; like the latter 

 they are arranged in about four layers, tljie outermost of 

 which is epithelial. At the lip of the blastopore the 

 outer passes into the inner sheet, the cells in the outer- 

 most layer of the former being gradually turned over 

 into the innermost layer of the latter. This inner sheet 

 also consists of several layers of cells, the innermost of 

 which is pigmented and epithelial, the remainder being 

 more irregularly disposed. The inner sheet forms the 

 outer, or, as it will be when the Qgg has rotated, the 

 upper wall of the slit-like cavity between itself and 

 the yolk-surface which is now covered up. This cavity 

 is the archenteron and the inner sheet of the fold is its 

 roof; its floor is the original vegetative surface of the 

 egg. This overgrowth and ingrowth of cells, with con- 

 sequent formation of an archenteric cavity, takes place 

 in an exactly similar manner at the lateral {Fig. 3, A) 

 and ventral {Fig. 2, E) lips ; and by the time that this 

 last has appeared the cavity extends over a considerable 

 area of the vegetative hemisphere, as far towards the 

 equator, in fact, as the line from which the lip of the 

 blastopore had originally begun to grow down. Now, 

 however, another process supervenes, namely, an upward 

 movement of the yolk-cells into the segmentation cavity. 

 The first indication of this is, as a matter of fact, 

 observable at an earlier stage (Fig. 2, B). Immediately 



