12 jE'iiKlNSON, Germinal Laj/ers of Vertebrates. 



has informed me, to be variable ; in a certain rather small 

 percentage of cases it is stated to fuse with the archen- 

 teron ; otherwise it is obliterated by further immigration 

 of yolk-cells ; traces of it may then be seen below the 

 archenteric floor [Fig. 2, F). 



The changes just described in the position of the 

 heavy yolk-cells afford the best explanation of the' rota- 

 tion of the egg ; for the centre of gravity being shifted to 

 the ventral side, the q^^ naturally turns about a horizontal 

 axis in the opposite direction. 



We are now in a position to discuss the formation ^ 

 of the germinal layers. With the exception of the yolk- 

 plug, the outer surface of the egg is entirely covered by a 

 sheet of small, pigmented cells, disposed in about four 

 layers, the outermost of which is epithelial. In part this 

 sheet consists of the original animal cells which formed 

 the roof of the segmentation cavity ; but it is also derived 

 in part from the outer sheet of the fold which grew down 

 at the lip of the blastopore. The sheet in question is of 

 course the ectoderm — as above defined ; dorsally it soon 

 becomes thickened to form the medullary plate. 



The notochord and mesoderm have a double origin. 

 Antero-ventrally — that is, in the region of the animal 

 hemisphere — they arise from the yolk-cells which have 

 been pushed and folded back into the segmentation cavity. 

 Postero-dorsally — that is, sub-equatorially — they are dif- 

 ferentiated in the inner sheet of the fold, the roof of the 

 archenteron ; the notochord in the mid-dorsal line in 

 front of the blastopore, the mesoderm in the remainder 

 of the roof around and behind the blastopore. The roof 

 consists of several layers of cells, the innermost or lowest 

 of which is epithelial ; the outer or upper layers consist 

 of fairly closely packed polyhedral elements {Fig. 4, A). 

 In the middle line (in front of the dorsal lip) a strip or 



