DESCRIPTION OF PLATES 
All of the photographie 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 
Ec., ectoderm 
En., entoderm 
E.Mes., embryonic or primitive streak 
mesoderm 
Em.N., embryonic nuclei 
Ec.Ves., ectodermal vesicle 
Ec.Cav., ectodermal cavity 
M.V., mesodermal vesicle 
P.G., primitive groove 
P.P., primitive pit 
Tr., Trager 
Tr.Ep., Trager epithelium 
Tro., trophoblast 
I, ventral embryo 
TT, right-lateral embryo 
IIT, dorsal embryo 
IV, left-lateral embryo 
G.C., giant cell 
Mes., mesothelium 
PLATE 1 
EXPLANATION OF FIGURES 
36 Median section of blastocyst No. 310. This shows Bee 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. 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. 
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