THE EMBRYONIC RUDIMENT 
33 
mass of cells. The amnio-embryonic cavity is excavated in the heart of 
the embryonic knob (Figure vi), and its floor, necessarily at first concave, 
and therefore apparently inverted or inturned, becomes the embryonic plate, 
while the roof becomes the amnion. The mesoblast appears very early 
and fills the space between the rudiment and the trophoblast. It is 
uncertain whether the amniotie lamella is from the first separated from the 
am. am.s. 
Figure VI. Hypothetical Stage op the Human Blastocyst (T. n. Bryce, 
prom “Quain’s Anatomy,” 11th Ed. Vol. I.). 
tr., trophoblast; am., amnio-embryonic cavity; am.s., amnion stalk; ent., 
entoderm. 
This diagram was designed to show, in one figure, the probable early condition 
of the trophoblast, and an early phase of the embryonic rudiment, but the size 
of the cavity of the blastocyst relatively to the size of the amnio-embryonic and 
entodermie vesicles, is probably rendered too small. We do not know how soon 
the trophoblast shell begins to assume dimensions so much out of proportion, 
as they appear in our ovum and in Peters’. The trophoblast is shown with a 
cellular and a plastnodial layer, but it is probable that when the plasmodium is 
at the stage represented, the embryonic rudiment is not so far differentiated as it 
appears in the diagram. With these qualifications the figure, which was printed 
before the present ovum was obtained, is probably not essentially incorrect. 
trophoblast. In Figure vi and Figure vm, p. 45, they are represented as 
remaining connected, because it seemed impossible, without the existence of 
such a connection, to explain the presence of the amnion duct seen in 
Beneke’s ovum, or the appearance of an open amnion as seen in Mali's 
pathological cases. While in man there is no true reversal of the ger¬ 
minal layers such as seen in mice and rats and the guinea-pig, there is 
E 
