THE DEVELOPMENT OF THE MOLE. 123 



represented in fig. 13 ; and here it will be seen, although there 

 is no positive division into somatic and splanchnic layers, yet 

 such a division is indicated in the section by the arrangement 

 of the nuclei of the cells on each side a line, which is repre- 

 sented by a narrow band of a lighter shade than the surrounding 

 tissue. 



In sections of three other embryos which I have examined, 

 about the centre of the medullary groove there is similarly an 

 indication of the splitting of the mesoblast from the periphery 

 to the axial portion, the cells being arranged in two parallel 

 rows along the inner edges of the two layers of mesoblast, 

 although no cavity is actually formed. Thus, although it 

 cannot be said that a split actually occurs through the whole 

 plate of lateral mesoblast in the Mole, yet there is without 

 doubt a tendency to such splitting in embryos of Stage e about 

 the centre of their body. 



In the same stage of growth (Stage e) is to be observed : 



(1) The separation of the axial and peripheral portions of the 

 mesoblastic plates, these two portions being connected by a 

 narrow neck of cells, the intermediate cell mass; and (2) 

 the formation of protovertebrse by means of clefts in the 

 axial mesoblast at right angles to the long axis of the embryo, 

 which divide this portion into cubical masses. The indication 

 of the splitting of the mesoblast at the same time becomes 

 more definite, and results in a cavity (fig. 15) within both the 

 protovertebrse and the peripheral mesoblast, a cavity which 

 does not, however, extend through the intermediate cell mass 

 (compare also fig. 17, Stage f). 



The cells of the protovertebrse are radially arranged round a 

 narrow elongated cavity, and form in a transverse section through 

 the middle of a somite a triangular mass, the apex of which is 

 situated at the base of the medullary groove. 



The cells of the peripheral mesoblast in the region of the 

 protovertebrse are columnar on their inner side and border, a 

 narrow slit extending to the periphery. At the edge of the 

 area the cells become flattened, and form a thin somatic and 

 thicker splanchnic layer, extending over the yolk-sac. 



