292 PROCEEDINGS OF THE AMERICAN ACADEMY. 



ectoderm. On the outside of the vesicle are a very few " Deckschicht" 

 cells. Beyond these facts the embryo presents no characteristic differ- 

 ences from the embryos already described. 



Second Stage. (Plate I. Fig. 1 ; Plate II. Fig. 10.) 



I will now take up a stage which shows a distinct advance in the 

 development of the embryo. Fig. 1, Plate I., shows the portion of 

 the blastodermic vesicle containing the germinal disk. This embryo 

 came from the uterus of the sow which was served before coming under 

 my control, so that I do not know the time which elapsed between 

 coitus and the killing of the animal. The stage of development, how- 

 ever, clearly places it at this point in my series. The whole vesicle, 

 which was somewhat wrinkled, was about 2.65 mm. in diameter, while 

 the germinal disk measured 0.205 mm. in its longest axis and 0.18 mm. 

 in its transverse axis, thus having an elliptical or slightly ovate outline, 

 which is shown in the figure. The drawing (Fig. 1) was made from 

 the whole object before staining, and is represented as seen by reflected 

 light against a black background. In sections the entoderm shows the 

 characteristic spindle-shaped cells in the extra-germinal region, while 

 in the area covered by the germinal disk it consists of cells lying closely 

 together and showing sharply defined cell boundaries. On the outside 

 of the blastodermic vesicle are a large number of " Deckschicht " nuclei 

 in the extra-germinal region, all with little chromatic substance and 

 with ill defined cell boundaries. But it is the ectoderm of the germinal 

 disk which presents the most interesting phenomena in this embryo. 

 Fig. 10, Plate II., shows a median longitudinal section through the 

 disk shown in Fig. 1, Plate I. Here the general surface of the disk is 

 seen to be somewhat depressed, thus leaving a raised margin, while at 

 one pole of the longest axis there is a large upgrowth or overgrowth 

 of ectodermal cells forming the bridge to which I have already re- 

 ferred. The beginning of the formation of this overgrowth is at what 

 I hope to establish as the posterior end of the germinal disk. The 

 bridge consists at this stage of practicalfy two layers of cells, which 

 are essentially the same in structure as the remaining ectodermal cells 

 of the germinal disk. Clearly this overgrowth is only a later stage in 

 the growth of the structure which was seen in its first stage of develop- 

 ipent in Figs. 7 and 8, Plate II., and the raised lateral margins seen 

 here I hold to be comparable with the raised margins described in the 

 previous embryos, while the depressed area seems to be due to a 

 broadening of the median groove of the vesicle, described in detail 

 above. It may be noted here that the germinal d;.sk is unusually 



