4 Jenkinson, Gerjuinal Layers of Vertebrates. 



understand that layer or set of cells which will give rise 

 to the epidermis and its derivatives, such as hair, skin- 

 glands, feathers, to the nervous system and organs of 

 special sense, and to the stomodaeum and proctodaeum ; 

 similarly by the " endoderm " I mean the cells which will 

 provide the lining epithelium for the alimentary tract and 

 its outgrowths, the liver, lungs, and so forth ; and by the 

 " mesoderm " the source of the musculature, skeleton, and 

 connective tissue, the blood and blood vessels and the 

 urino-genital system. 



THE FROG. 

 In the Frog the process of germ-layer formation 

 begins when segmentation is completed. The fully 

 segmented ovum consists of an animal hemisphere (rather 

 more than a hemisphere) of small pigmented cells, con- 

 taining but little yolk, and a vegetative hemisphere 

 of bulky white cells full of large yolk-granules. From 

 the centre of the animal hemisphere (or animal pole) a 

 straight line can be drawn passing through the centre of 

 the egg and the centre of the vegetative hemisphere (the 

 vegetative pole) ; this line is the egg-axis and about it the 

 ^gg is radially symmetrical. In the normal position of 

 the Q.oa the axis is vertical. Whether, as Schulze has 

 maintained, segmentation has already conferred a bilateral 

 symmetry upon the &^^ is a point which for our present 

 purpose is immaterial. At the equator of the ^^^ are 

 cells whichiare in every respect intermediate between the 

 small animal and the large vegetative cells. In its interior 

 is a spacious segmentation cavity, hemispherical in shape, 

 placed axially but excentrically and nearer the animal 

 than the vegetative pole. Its roof is formed of about four 

 layers of small cells, the outermost of which is arranged 

 very distinctly as a cubical or shortly columnar epithelium^ 



