DESCRIPTION OF PLATES 



All of the photographic figures shown in these plates were made directly from 

 the negative without retouching. The photographs were made by the use of the 

 Bausch and Lomb combination micro-photographic and drawing apparatus. 

 Various combinations of Zeiss objectives and oculars were used in the work. In 

 practically every case the magnification is given after the description of the figure. 



ABBREVIATIONS 



C.A.V., common amniotic vesicle M.V., mesodermal vesicle 



EC., ectoderm P.O., primitive groove 



En., entoderm P.P., primitive pit 



E.Mes., embryonic or primitive streak Tr., Trager 



mesoderm Tr.Ep., Trager epithelium 



Em.N., embryonic nuclei Tro., trophoblast 



Ec.Ves., ectodermal vesicle /, ventral embryo 



Ec.Cav., ectodermal cavity 77, right-lateral embryo 



G.C., giant cell ///, dorsal embryo 



Mes., mesothelium IV, left-lateral embryo 



PLATE 1 



EXPLANATION OF FIGURES 



36 Median section of blastocyst No. 310. This shows the knob-like inner cell- 

 mass lying against the thin trophoblast. X 186. 



37 Median section of blastocyst No. 296. The deeply staining entodermal 

 cells are seen in the act of migrating to the lower or inner side of the cell-mass. 

 X 186. 



38 Median section of blastocyst No. 335. Note that the trophoblast has not 

 yet become thinned out or attenuated. X 186. 



39 Enlarged view of the inner cell-mass of the blastocyst shown in the 

 next figure. It clearly shows the pseudopod-like processes extending out from 

 the marginal entodermal cells. X 225. 



40 Upper view of blastocyst No. 320. From an unstained glycerin jelly prep- 

 aration. X 90. 



41 Median section of blastocyst No. 249. The section does not lie flat on the 

 side, and since the microscope was focused on the trophoblastic cells, the inner 

 cell-mass is slightly out of focus. Note how sharply Rauber's layer stands out 

 from the mass. X 417. 



42 Enlarged view of the inner surface of the entoderm of blastocyst No. 300. 



662 



