DE. E. B. WILSON ON THE DEVELOPMENT OE RENILLA. 
761 
The oesophagus remains in this condition for a considerable period (twenty to twenty- 
five hours), during which the only change consists in the clear definition of the cavity 
and the expansion of its lower end (fig 136). In most cases the lower angles of the 
cavity are prolonged downwards so that the cavity has a distinct Y-shape ; this form is 
sometimes much more pronounced than in the figure. The cavity then breaks through, 
thus placing the gastric cavity for the first time in communication with the exterior. 
I have made many sections, longitudinal and transverse, through the oesophagus at 
this period, a study of which leaves little doubt that great variation exists in the 
formation of the mouth, as in so many other features of the development. The most 
common mode is illustrated by figs. 137-140. The wall of the oesophagus thins away 
by absorption at one of the lower angles of the Y (fig. 131) and finally breaks away 
at this point (fig. 139). At the opposite side the mass of tissue forming the bottom 
of the oesophagus still remains attached to the lateral wall of the oesophagus and to 
the edges of the septa which have meanwhile been formed. As the septa grow 
backwards this mass of tissue (which for the sake of convenience I shall call the 
oesophageal plug} is carried down with them, being attached to the edges of one or 
more of them (fig. 140), sometimes by a narrow neck. The mass of tissue is then 
gradually absorbed and the oesophagus is left in free communication with the gastric 
cavity. In several of my specimens, at this stage, a large mass of tissue may be 
observed lying in the gastric cavity below the oesophagus. This is quite similar in 
appearance to the oesophageal plug, and is, I believe, identical with it. Hence it 
would appear that in some cases absorption takes place all around the oesophageal 
plug, which finally drops out bodily into the gastric cavity and is there absorbed as if 
it were food or yolk-material.- In a number of specimens, one of which is shown in 
fig. 141, a still different mode was observed. Absorption here begins near the middle 
of the bottom of the oesophagus between the two arms of the Y~ s h a P e d cavity and 
the opening at length breaks through at this point, leaving the remains of the 
oesophageal plug attached to the lips of the oesophagus where they are absorbed. 
During these changes the layer of ectoderm forming the bottom of the oesophageal 
cavity becomes indistinct, and in most cases the supporting lamella which separates it 
from the underlying mass of entoderm disappears. The cells of both layers in the 
plug change their character and are no longer differentiated by the staining fluid, so 
that the oesophageal plug appears to be composed of uniform confused granular cells. 
In one of my specimens (fig. 159) the greater part of the oesophageal plug seems to 
have been absorbed, leaving the supporting lamella stretching across the oesophageal 
cavity. Below this is a mass of delicate debris , which is apparently the last remains 
of the oesophageal plug. 
Review. 
The earlier view, according to which the oesophagus is to be regarded as a stomach, 
opening below into the body cavity, is now entirely abandoned. In view of its 
