THE EMBRYONIC RUDIMENT 
31 
The mammalian blastocyst is a hollow vesicle with a knob of cells 
projecting into its cavity from one point on the wall (Figure iv). The 
ectoderm of the wall or trophoblast is concerned in the processes of 
imbedding and placentation, the inward projection constitutes the embryonic 
blastema. Whereas in the rabbit the cells forming the embryonic knob 
become spread out flat at the upper pole of the blastocyst (Figure v, a), 
and are soon exposed on the surface, by the disappearance of the thinned 
out trophoblastic covering (Rauber’s layer), in another series of mammals 
the knob remains inturned and a cavity appears among the cells of 
Figure IV. Blastocyst of Tarsius Spectrum (after Hubrecht), from 
“Quain’s Anatomy,” 11th Ed. Vol. I. Embryology. 
the knob. In some cases the roof of this cavity early breaks away 
(Figure v, b ), and the embryonic ectoderm, becoming flattened out, is 
exposed on the surface just as in the rabbit. In other instances the roof 
of the cavity persists and forms the definitive amnion (Figure v, e, f). 
There is very good reason for believing that this is the case in the human 
subject. Owing in all probability to the nature of the processes of 
imbedding, the embryonic knob or formative-cell mass remains in its 
original position, and the trophoblast becomes uniformly thickened all over 
the sphere. The embryonic rudiment lags behind in development, and 
there is a relatively enormous expansion of the trophoblast shell, which 
is concerned at first in the excavation of the implantation cavity and 
then in the nourishment of the embryo. 
The entoderm is probably split off, to judge from the stages in 
