56 PROCEEDINGS OF THE AMERICAN ACADEMY 



characteristics before mentioned. At this time (Fig. 4) the surface 

 cells, which are destined to form the epidermal layer, are more flat- 

 tened than the deeper cells. It is not certain, however, that this layer 

 is now distinct from the deeper cells. There are some very good 

 grounds for thinking that some of the deeper cells eventually take a 

 superficial position, assuming the flattened form. 



Three hours after the 16-cell stage, we find a wreath of flattened 

 cells encircling the blastodisc. Faint boundary lines are sometimes 

 visible in the living ovum, but they soon disappear. Sections of the 

 blastodisc at this time show that the wreath is composed of two con- 

 centric rows of cells, the inner of which lies beneath the margin of 

 the cap, and hence is not easily seen in the living ovum. The inner 

 row of cells is continuous with the thin periblastic floor of the cleavage- 

 cavity ; and the outer row is continuous with the larger external 

 periblast. These cells (Fig. 5), derived from the marginal cells of 

 preceding stages, are quite distinct from all others of this stage, if 

 we except one or two usually seen above them ; and, as they form 

 the Anlage of the periblastic cell-layer, they may be called periblastic 

 cells. 



The one or more cells lying above the periblastic cells, which are 

 also shaded in the figure, are less strongly individualized ; but are, in 

 many (not all) preparations, easily distinguished from the overlying 

 cells. These cells, which we regard as the Anlage of the future 

 entodermic layer, differ from the superjacent cells less than from the 

 periblastic cells. 



The periblastic cells multiply rapidly, the nuclei passing through 

 the typical amphiastral phase at each division. As they increase iu 

 number, they spread in both directions, — .inward, beneath the blasto- 

 disc, and outward, beyond the margin of the same, — thus forming the 

 syncytial layer described by Kupffer, Van Beneden, Ryder, Hoffmann, 

 and others. 



When the entodermic ring begins to form (Fig. 6, en), we find that 

 the periblastic nuclei have already spread far under the blastodisc. 

 At this time there are no cell-boundaries around these nuclei, so 

 far as can be ascertained from sections and mounted preparations. 

 The periblast is now only a little thicker under the outer edge 

 of the blastodisc than elsewhere; and no nuclei are found com- 

 pletely outside this thickened portion (" bourrelet peripherique," Van 

 Bambeke). 



The periblast then becomes a cellular layer as the result of one 

 of the concluding acts of cleavage. The so-called "free nuclei" 



