290 PROCEEDINGS OF THE AMERICAN ACADEMY. 



on the inner surface of the ectodermal layer, and are more numerous in 

 the region of the germinal disk than in tlie extra-germinal area, but they 

 everywhere retain their elongated flattened outline. Tlie germinal disk 

 was cut into ten sections, of which Figs. 7 and 8 represent respectively 

 the second and third, and Fig. 9, the sixth. According to my method 

 of orientation the two former are at the posterior end of the future 

 embryo, the latter nearer the anterior end. It will be seen from the 

 figures that the ectodermal cells of the germinal disk are slightly elon- 

 gated in a direction at right angles to the surface of the disk, and 

 show a tendency to arrange themselves in two interlocking layers. At 

 the margin of the disk the transition from disk ectoderm to extra- 

 germinal ectoderm is abrupt, but the general characteristics of the 

 cells in the two regions is such as to suggest an identity of origin at 

 least, — the differences being merely the slight variation in the chro- 

 matic affinity of the cytoplasm, which I have already mentioned, and 

 the change in shape, which would necessarily attend a difference in 

 cell arrangement. The exposed surface of the disk is pretty uniformly 

 even, except at one very significant point. Two sections through this 

 point are shown in Figs. 7 and 8, Plate II., where in the centre of 

 each section a cell is seen to project above the general level of the 

 disk, and in Fig. 8 to be slightly cut off from it. The same appear- 

 ance is present in the two sections which intervene between Fig. 8 and 

 Fig. 9, and the whole projection running through the four sections 

 contains three cells, which are without any question true ectodermal 

 cells of the germinal disk, which have assumed this new position. 

 These cells represent a very early stage in the development of a 

 structure which I shall later designate as the bridge, and for that 

 reason 1 shall call these bridge cells. It should be noted here that 

 there is no evidence whatever of a third layer of cells, i. e. a " Deck- 

 schicht," outside the ectoderm in the region of the germinal disk, 

 though in the extra-germinal region there are a few widely separated 

 cells attached to the outer surftice of the ectoderm ; these differ from 

 the ectodermal cells in having very small nuclei ; they resemble the 

 cells of the entoderm in general outline. By an enumeration of the 

 nuclei I have ascertained approximately the number of cells in the 

 area of the germinal disk, which is as follows : cells in the ectoderm 

 proper, 188; in the entoderm, 48; in the bridge, 3. The presence 

 and position of this bridge supply the criterion on which I have deter- 

 mined the chief axis and its poles for the future embryo. It should be 

 further noted here, that in the two sections immediately succeeding 

 Fig. 9, i. e. sections seven and eight of the series through the disk, 



