GERMINAL LAYERS AND EARLY DEVELOPMENT 0¥ MOLE. Ill 



described specimens lay above the plate ; the flattened cells 

 of this part soon become columnar and the fusion between 

 them and the plate becomes complete. 



Somewhat prior to this stage the edges of the plate 

 become continuous 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 Lieber- 

 kiihn 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 blastodermic 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 ' Com- 

 parative 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) conjointly from the at first 

 flattened cells of the primitive outer layer (called by Rauber 

 and KoUiker '^ 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 consists of a two-layered and a single- 

 layered portion ; the latter, formed of epiblast, alone com- 



