ON THE FORMATION OF THE GERMINAL LAYERS IN PETROMYZON. 135 



but smaller and less tall than the hypoblast cells. Then- cell-limits 

 are also more distinct, Tlie floor of the invaginated tube is formed by 

 the upper row of y(jlk-cells, which have assumed a more or less 

 columnar shape. 



I wish to emphasize liere the fact that the invaginated tube is not 

 the same as the cavity ^of the future mesenteron. As the subsequent 

 history shows, the whole mass of yolk-cells is absorbed with the 

 exception of the outermost row lying immediately inside the epiblast. 

 This latter l)ecomes the definitive hypoblast (fig. 27 and fig. 19). 

 Substantially tlie same tiling has been shown ])y Scott (/. c. pp. 121- 

 122), except that he regards the outermost row as destined to form the 

 mesoblast, and the row next inside as forming the definitive hypoblast. 

 AVhen once we remember the above fact, one can easily perceive that 

 what Kupffer belicNes to be the liver, in his figs. 27 and 28 (e), could 

 not be such. This org;in can notarise among the cells which are after- 

 wards absorbed. 1 am inclined U) think that what he calls the liver 

 in liis fig. 28 is the slit which tu'ises between the main body of yolk- 

 cells and its outermost row (cf fig. 27). 



ÏÎ. The Formation of the Mesoblast. 



At about the time when the gastnilation is ahnost completed, the 

 eo-o' underofoes one (^i' the most important changes. To the two 

 primary layers already dealt with, a, third layer, the mesoblast, is 

 added. The formation of this layer forms one of the most difticuh 

 subjects in the study of the embryology of Pelroiwjzon. The altera- 

 tions at first met with are altogether in the internal structure and can 

 therefore be made out only in sections. 



