DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 447 
orifice is closed, from the front of the urino-genital sinus to 
the umbilicus. The ventral extension of the posterior part of 
the celom is not interfered with by the early appearance of 
the allantoic mesoblast; therefore the separation of the 
splanchnopleure proceeds rapidly, not only anteriorly and 
laterally, but also posteriorly, and the yolk-sac is soon com- 
pletely separated from the trophoblast. 
If this explanation is correct there is no necessity for the 
trophoblastic covering of the epiblastic area which Hubrecht 
has supposed to be present in the human ovum, but of which 
there is no trace in the youngest known specimens (Reichert’s, 
as figured by His), nor for retardation of mesoblastic cleavage 
in the posterior part of the embryonic region. The union of 
the embryo with the chorion is not maintained by means of 
the tail amnion, along which the allantois grows, as Hertwig 
suggests, but by union of the allantoic mesoblast with the 
chorion before the central part of the tail amnion fold is 
formed, and the “ Bauchstiel’’ is not entirely independent of 
allantoic formation, as His asserts (21, p. 172): for although 
the “ Bauchstiel ” is not the allantoic stalk, it contains all the 
characteristic constituents of that stalk, the hypoblastic tube, 
the arteries, and veins; and it is the posterior portion of the 
embryonic area to which, as in all mammals, the intra-embryonic 
portion of the allantoic stalk is attached: further, it takes 
part, during the later periods of embryonic life, in the forma- 
tion of the posterior portion of the ventral wall of the embryo. 
The Mesoblast. 
A complete consideration of the peculiarities of mesoblast 
formation in the rat and the mouse, and a comparison of 
them with the main facts which have been noted in other 
Vertebrates, must be deferred to a subsequent communication. 
For the present I merely desire to draw attention to the fact 
that the phenomena observable in the rat and the mouse during 
the development and extension of the middle germinal layer 
point to the conclusion that this layer is developed from the 
peristomal cells and from the hypoblast. The mesoblastic 
