246 RICHARD ASSHETON. 



hypoblast of the meroblastic egg, then a distinction might 

 perhaps be drawn between true hypoblast and trophoblast, and 

 compared witli the true hypoblast and yolk nuclei of the bird's 

 egg, by whose union in the bird's egg the bourellet endo- 

 dermique is formed (Duval). 



The Relation of the Allantois to the Amnion. 



In a former paper (1) on the early stages of the develop- 

 ment of the rabbit I ascribed the apparent extension of the 

 hypoblast round the inner surface of the blastocyst to the 

 presence of a region of special activity in the extra-embryonic 

 epiblast, or, as I would now rather call it, the trophoblast, 

 which begins to be manifest as early as the fifth day, and cul- 

 minates at last " in the production of the ectoplacental area, 

 or part of it'' (p. 146). When discussing in a subsequent 

 paper (3) the mode of extension of the primitive streak meso- 

 blast, 1 was not able to understand how the extension of that 

 layer in the form of a wide-meshed reticulum, such as it is in 

 its peripheral parts, could be produced except by its being 

 dragged away from the primitive streak by the expanding 

 walls of the blastocyst in a way analogous to that assigned for 

 the spreading of the hypoblast. 



Accordingly I thought that the overlying layer must also 

 have come from the primitive streak, and wrote, " We may 

 take the outline of the primitive streak mesoblast as indicat- 

 ing also the outline of the epiblast derived from the primitive 

 streak, at any rate for the stages up to about the one drawn 

 for fig. 6. At this time cells are separated from the hypo- 

 blast, both at points where there is already existing primitive 

 streak mesoblast, and at points where., up to now, there has 

 been no mesoblast.'' So far, I think, I was right; but in 

 drawing the diagrams on PI. 22, and in writing the descrip- 

 tion of them, I seem to have neglected to consider the meso- 

 blast of hypoblastic origin in the region round the primitive 

 streak (originally described by Henseu), and so fell into the 

 error of taking the extreme outline of the mesoblast as an 

 indication of tlie limit of cells of primitive streak origin at this 



