194 Mr. W. Heape. On the Germinal Layers [Dec. 22, 



plate; the flattened cells of this part soon become columnar, and 

 the fusion between them and the plate below becomes complete. 



Somewhat prior to this stage, the edges of the plate become con- 

 tinuous with the outer layer of the wall of the vesicle beyond the 

 region of the embryonic area. 



Thus, the greater part of the inner mass of cells, as Lieberkiihn 

 correctly states, combines in the form of a plate of more or less columnar 

 cells with that part of the flat outer layer of cells which immediately 

 overlies it, to form a plate of columnar cells two and three rows deep ; 

 this plate is the epiblast plate of the embryonic area, the remainder 

 of the outer layer of cells forming the epiblastic wall of the blasto- 

 dermic vesicle. 



The portion of the inner mass of cells which was separated off 

 from the main mass as the hypoblast still forms only a single row of 

 somewhat rounded cells, the central part of which underlies the 

 embryonic area, while the peripheral part continually extends as a 

 layer of flattened cells along the inner aspect of the epiblastic wall of 

 the remainder of the blastodermic vesicle. 



In concluding this portion of my subject I may add, in support of 

 what I believe in harmony with Lieberkiihn to be the origin of the 

 true epiblast of the embryonic area in the mole, that in the course of 

 my work on this subject, carried on since the investigations of Mr. 

 Balfour and myself, published in the second volume of Mr. Balfour's 

 " Comparative Embryology," I have obtained from an embryo rabbit 

 of six days four hours old, sections which appear to me conclusively to 

 confirm the results at which we before arrived, namely, that the 

 epiblast plate of the embryonic area is derived (as in the mole) con- 

 jointly from the at first flattened cells of the primitive outer layer 

 (called by Rauber and Kolliker " Deckzellen," and stated by those 

 observers to disappear from the embryonic area), and from the larger 

 portion of the primitive inner mass of cells (held by Van Beneden to 

 be the true mesoblast, and stated by Kolliker alone to form the 

 epiblast plate). In the sections of this embryo the cells of the 

 already described primitive outer layer are seen in a transition stage, 

 being wedge-shaped and prolonged in between the cells of the inner 

 mass. 



At the stage of growth now arrived at the blastodermic vesicle 

 may be considered to consist of an embryonic and a non-embryonic 

 portion. A surface view shows the embryonic area to be in the form 

 of a more or less circular opaque disk. The wall of the vesicle con- 

 sists of a two-layered and a single-layered portion ; the latter, formed 

 of epiblast, alone comprises the portion of the wall opposite to the 

 embryonic area, while the former, consisting of epiblast and hypoblast, 

 forms the embryonic area and the part of the vesicle immediately 

 -adjoining it. 



