EARLY STAGES OF DEVELOPMENT OF THE RABBIT. 151 



iuuer mass cells re-assume, at any rate ou their free surfaces, 

 the rounded contour which is natural to them. 



As the blastodermic vesicle expands, the inner mass, which 

 is adherent to the wall of the vesicle either by actual proto- 

 plasmic connections or otherwise, is drawn out into a lenticular 

 shape. 



I have tried to show that there is a zone of the wall of the 

 vesicle which, by greater activity of multiplication of cells, 

 admits of more rapid expansion of that part. Upon this zone 

 rests the edge of the lenticular inner mass. The expansion of 

 the zone is in direction radially from the embryonic poles. 

 Hence the outermost cells of the inner mass, i. e. the cells at 

 the edge of the lenticular mass, will tend to be separated more 

 rapidly than the innermost. This will tend to isolate these 

 cells from others of the inner mass. Let us suppose that all 

 the cells of the hypoblast layer are dividing at a uniform rate. 

 I think it is reasonable to suppose that the existence of strands 

 connecting cells of this kind together have their origin in 

 past cell divisions. Accordingly on this supposition the con- 

 necting strands will be more numerous and the nuclei nearer 

 together, and the meshes of smaller area in the embryonic disc 

 hypoblast than in the hypoblast outside that area. The hypo- 

 blast cells of the embryonic area will differ from those of the 

 extra-embryonic area in this way : 



(i) The embryonic cells will have more and shorter, and so 

 presumably stronger, strands connecting them with their neigh- 

 bours than will the extra-embryonic hypoblast cells. 



(ii) The embryonic hypoblast cells will have connecting 

 strands upon all sides, whereas the outermost extra-embryonic 

 cells will have them upon one side only. 



Is it possible for the flattening of the embryonic hypoblast 

 cells to be due to their becoming stretched by the tension 

 produced by the extra-embryonic hypoblast cells (with which 

 they are in direct connection by means of the strands just men- 

 tioned) being removed in all directions by the rapidly expanding 

 zone of the outer epiblast? If so, it is possible to account for 

 all the shapes of the cells composing the embryo at this age. 



