42 WALTER HEAPE. 



mass remaining the same in size, and attached to the outer layer 

 in the region of the now-closed " blastopore." Subsequently the 

 inner mass flattens out and splits up into two layers, the lower 

 of which forms the hypoblast and the upper the mesoblast of 

 the embryonic area, the epiblast being formed solely by the 

 multiplication of the outer layer cells, which become at the 

 same time columnar and arranged in a single row. 



Beneden, therefore, has not found a stage in which two 

 layers only exist throughout the area. 



Rauber (No. 21), in a previously written paper, finds three 

 layers present in the embryonic area of a rabbit before the 

 formation of the primitive streak ; the outer of these (my outer 

 layer) he calls the " Deckschicht," and states that it early dis- 

 appears, while the middle layer alone forms the epiblast and 

 the lower the hypoblast of the area ; a two-layered area being 

 thus formed. 



Kolliker, in a recent elaborate paper (No. 16), traces the 

 fate of the three layers, which he also finds in common with 

 Rauber and Beneden, and declares, in accordance with the 

 views of the former author, that the outer layer gradually dis- 

 appears, the middle forming the epiblast and the lower the 

 hypoblast of the embryo. The details of the gradual disap- 

 pearance of the Deckschicht occupy much of this paper. Pro- 

 fessor Kolliker has never seen the cells of this layer assume 

 a columnar form, as Beneden asserts is the case, and by 

 means of nitrate of silver staining he satisfies himself they 

 gradually become broken up, and eventually disappear al- 

 together. 



Lieberkiihn (No. 19) gives an account of the formation of 

 the epiblast in the dog and mole which is very similar to my 

 own, in that he considers it is formed of the greater portion of 

 the inner mass, together with that portion of the outer layei 

 cells which originally overlaid it. He also draws attention to 

 the cavity which appears, according to him, within the inner 

 mass of cells in the mole, and which he suggests may be com- 

 parable to the segmentation cavity of other animals. 



Hensen (No. \2) for the rabbit, and Schafer (No. 25^ for Ihe 



