DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 445 
no part in its formation. Hertwig (20) suggests that the 
** Bauchstiel ” is nothing more than the allantoic stalk which 
has been conducted backwards to the chorion by the posterior 
part of the amnion. Hubrecht has entered upon an elaborate 
explanation of its formation (24). He suggests that in the 
human ovum the epiblast disc is covered by a layer of tropho- 
blast to which its margins are adherent, as in the hedgehog. 
As the amnion folds rise the margins of the epiblast are 
gradually separated from the trophoblast, but at one point the 
connection remains permanent, and so the continuity of the 
embryo and chorion is maintained. The mesoblast growing 
backwards from the primitive streak extends beneath this epi- 
blastic connection. This mesoblast, being axial, remains 
unsplit until the mesoblastic connection between the embryo 
and the chorion is established, and the splanchnopleuric layer 
is afterwards separated. 
Neither the trophoblastic covering of the epiblastic area, 
nor the delayed splitting of the posterior portion of the meso- 
blast, seems to be an essential factor in the production of the 
conditions found in early human ova. 
Commencing with the youngest ovum figured by His (21, 
fig. 17 a, p. 171), in which the epiblastic disc is not covered by 
the trophoblast, but is continuous with that portion of the 
external layer by its margin, we may imagine that mesoblast 
formation takes place early, but in the usual way—that is, it 
commences posteriorly, and extends laterally round the mar- 
gins of the epiblastic area to its anterior extremity. After the 
mesoblast is well formed, we may suppose that the splitting of 
the layer commences bilaterally and posteriorly as in the rat 
and the mouse. As the splitting extends inwards through the 
posterior part of the axial mesoblast, it separates the somato- 
pleure from the splanchnopleure. (This separation evidently 
occurs at an early period in the human ovum, for from the 
first the ccelom intervenes between the under surface of the 
“Bauchstiel ” and the posterior wall of the yolk-sac.) Whilst 
the splitting is proceeding in this region, and before the dis- 
tension of the colom occurs, the allantoic mesoblast grows 
