294 PROCEEDINGS OF THE AMERICAN ACADEMY. 



than in either the stage under discussion or that represented by the 

 figures on Plate III. 



The cells of the entoderm are essentially the same in structure in 

 the region of the germinal disk as they were over the same area in the 

 embryo represented in Fig. 1, Plate I., and Fig. 10, Plate II.; but 

 there is a further thickening or increase in number of the entodermal 

 cells immediately surrounding the germinal disk, and extending on all 

 sides of it for a distance about equal to the diameter of the disk itself, 

 so that the diameter of the entodermal thickening is at this stage about 

 three times the diameter of the germinal disk. The normal distribu- 

 tion of the entodermal nuclei in the remainder of the extra-germinal 

 area can be seen from Fig. 26, Plate IV. A possible explanation as 

 to the significance of this thickening of the entoderm has occurred to 

 me, which I will mention in the theoretical considerations concerning 

 the interpretation of the vesicle. 



Fourth Stage. (Plate I. Figs 3-5, Plate II Figs 11-13; Plate III. 

 Figs 14-19 ; Plate IV. Fig. 20.) 



The next two embryos in the series are shown in Figs. 3 and 4, 

 Plate I. I have given no sections of these, as the surface views show 

 the more important characteristics, and because they are essentially the 

 same as sections of other embryos which I have figured farther on. 

 Fig. 3 shows that the bridge has a nearly circular margin bordering 

 the opening into the cavity beneath. There is no evidence on the sur- 

 face of the germinal disk or in the sections — which are taken at right 

 angles to the longer (i. e. transverse) axis — as to whether the bridge 

 arose at three points, as mentioned above, and then became one by a 

 fusion of the three parts, or whether it arose as one continuous over- 

 growth from the margins towards the centre of the disk. The vesicle 

 to which this germinal disk belongs was 2.25 mm. in diameter and very 

 little folded. The disk itself measured 0.19 mm. in its longer axis and 

 0.15 in its shorter or antero-posterior axis. It will be observed that 

 tlie elliptical outline of the germinal disk is the same with regard to 

 the chief axis of the future embryo, as in the case of the first embryo 

 described. 



Figure 4 represents a disk which is a little older than that shown in 

 Fig. 3. The bridge covers a larger portion of the germinal disk than 

 in the preceding case, and if we assume that the disk had at first the 

 shape shown in Fig. 3, it has begun to elongate in the direction of its 

 chief axis, until it is now nearly circular in outline. The right-hand 

 side of the figure shows a lateral opening into the cavity beneath the 



