EARLY STAGES OF DEVELOPMENT OP THE RABBIT. 137 



day the cells of the outer layer may be almost circular, or may 

 have any number of sides curved or straight. 



Changes in the inner mass. — These are very important 

 during the fifth day. 



Up to now the cells of the inner mass have been very uni- 

 form in character. During the process of flattening of the 

 mass (a stretching out as above suggested) the individual cells 

 have likewise become flattened, and each is somewhat lenticular 

 in shape (figs. 27 and 28). 



The inner mass presents an approximately circular outline 

 when viewed from above or below, as in fig. 38, but here and 

 there single cells [HY. I.) seem to jut out somewhat beyond the 

 others. So we may say at this moment that the embryonic 

 disc is several layers of cell thick at the centre, but thins out 

 toward the periphery (figs. 26, 27). 



The whole embryo is now spherical. Early on the fifth 

 day, somewhere, as a rule, between the 96th and 100th hours, 

 the cells which were noticed before as jutting slightly 

 from the sides of the " inner mass " may now be seen to be 

 quite separated from it, as in the diagram fig. 39, HY. /., 

 standing out clear and round. These are not easily seen in 

 the perfectly fresh specimen, but are brought out beautifully 

 by certain reagents, as, for instance, Flemming's strong solu- 

 tion, or a filtered mixture of one part Perenyi with one part 

 picro-carmine ; indeed, almost any reagent that stains slightly 

 the cells and not the albuminous layer. 



At the same time the cells of the centre of the inner mass 

 have become more flattened, and can now be seen to form a 

 patch of cells, nowhere more than two cells thick. That is to 

 say, the embryonic area is composed of, in all, three layers of 

 cells, the outer a definite membrane continuous with the 

 general wall of the vesicle, and two other looser layers of cells 

 slightly flattened but much thicker and rounder than the cells 

 of the outer layer. 



Beyond the periphery of this mass, a number of cells very 

 much rounder may be seen scattered irregularly over the inner 

 surface of the thin outer layer, extending over an arc of 



