426 WALTER HEAPE. 



cat, also describe a two-layered stage of the embryonic area 

 prior to the formation of the primitive streak. 



Summary. — With regard to my own work I hold that the 

 blastodermic vesicle increases in size, not merely on account of 

 the increase in number and the flattening of the outer layer cells, 

 as Beneden believes, but by the migration of inner mass cells to 

 the exterior. This view is supported by the fact that the inner 

 mass decreases in size during the early devrlopment of the 

 vesicle. I have also satisfied myself of the existence, both in 

 the mole and rabbit, of a stage in which the embryonic area is 

 composed of only two layers, the epiblast and hypoblast. 



The hypoblast I have shown to be derived from the cells of 

 the inner mass — a fact which all the observers above mentioned 

 are agreed upon. 



The epiblast I believe to be formed, as does Lieberkiihn, of 

 the remaining portion of the inner mass, after the hypoblast 

 has been detached, together with that portion of the outer layer 

 which overlies the inner mass. 



In the mole this includes also certain cells which we have 

 seen are derived from the outer layer, ajid which at one time 

 lie in a cavity between that layer and the inner mass.^ In the 

 rabbit, however, no such cells exist, and I believe that the epi- 

 blast is formed of inner and outer layer cells. 



With reference to the development of the epiblast in the rabbit 

 I may saythat since working at the question under the supervi- 

 sion of the late Professor Balfour (No. 3), I have examined more 

 embryos, and have been fortunate enough to obtain good sec- 

 tions of the embryonic area of a rabbit embryo of six days four 

 hours old, which appear to me to be conclusively in favour of 

 the view we were then inclined to accept. Fig. 49 represents 

 a section through this area ; in it the epiblast plate is seen to 

 be composed of two entirely different kinds of cells — (1) a 

 lower more or less columnar or rounded cell, and (2) an 

 upper flattened or wedge-shaped cell. The latter cells in- 

 variably occupy a position on the outer side of the plate, across 



• In a previous paper (No. 11) I erroneously described these cells as being 

 derived from the inner mass. _ 



