354 G. CARL HUBER 
By the end of the fifth day after insemination, all fertilized, 
normal ova are found in the blastodermic vesicle stage. One 
pole of each vesicle, its floor, consists of a relatively thick mass 
of cells, in which there is no differentiation in layers and no 
evidence of ectodermal and entodermal cells. The other pole. 
of each vesicle, its roof, consists of a single layer of flattened 
cells, bordering the segmentation cavity. 
During the sixth day, the blastodermic vesicles which still 
lie free in the lumen of the uterus, increase in size, partly as a 
result of extension of the roof cells, partly owing to rearrange- 
ment and flattening of the cells of the floor. This portion of 
the vesicle now presents the form of a concavo-convex disc, 
forming about one-sixth of the vesicle wall and consisting, as a 
rule, of three layers of cells, the inner of which is now differen- 
tiated to form the yolk entoderm. 
During the seventh day after insemination the blastodermic 
vesicles become definitely oriented in a decidual crypt, the 
thicker portion, its floor, being directed toward the mesometrial 
border. The phenomenon of the “inversion of the germ layers’’ 
or ‘‘entypy of the germ layers’ is initiated, the result of cell 
rearrangement and cell enlargement in the germinal disc, mani- 
fested as an outgrowth to form the ectoplacental cone or Trager 
and an ingrowth into the vesicle, the anlage of the egg-plug or 
egg-cylinder. In the egg-plug there is recognized a circum- 
scribed, compact mass of cells, staining more deeply than sur- 
rounding cells, which constitute the ectodermal node, the anlage 
of the primary embryonic ectoderm of the future embryo. This 
ectodermal node, so far as it extends into the cavity of the 
blastodermic vesicle, is surrounded by yolk entoderm. 
During the eighth day after insemination, the egg-cylinder 
comes in definite relation with the maternal decidua and re- 
ceives as embryotroph maternal hemoglobin, partly through 
phagocytic action of the cells of the ectoplacental cone, partly 
through absorption of maternal hemoglobin by the cells of the 
entoderm, initiating a period of very active growth as evidenced 
by active mitosis. The egg-cylinder increases in length, and 
entypy is completed. A cavity develops in the ectodermal 
