DE. E. B. WILSON ON THE DEVELOPMENT OP RENILLA. 
783 
of each is occupied by a delicate supporting lamella, continuous outwardly with that of 
the body-wall, and inwardly with that which separates the ectodermic and entodermic 
layers of the oesophagus. The entoderm cells are arranged upon both sides of the 
lamella in a thick irregular layer. They are of an elongated pyriform shape, and are 
so large and closely packed as to fill up entirely, in most cases, the spaces between the 
septa. In one or two of the compartments, however, a small space appears near the 
middle, the cells radiating towards it in all directions from the septa, body-wall, and 
oesophagus. These spaces constantly increase in size as development proceeds, and 
form the radiating chambers which surround the stomach. The cells are so closely 
packed at first that a longitudinal section in nearly any plane gives the appearance of 
fig. 136, the entoderm having the appearance simply of being greatly thickened in the 
oral region. 
Anteriorly the septa extend quite across the gastric cavity from the oesophagus to 
the body-wall, as shown in the figure. Behind the oesophagus their inner edges are 
free and the septa appear in transverse section as low ridges which scarcely rise above 
the level of the general layer of entoderm. They may, however, be readily recognised 
by the presence of the central layer of supporting lamella and the radiating disposition 
of the cells over them. This is shown in fig. 143, which represents a section from the 
same larva (forty-eight hours) with fig. 142 taken farther back at the lower end of the 
oesophagus. Three of the septa still reach the oesophagus («?.), two are barely united 
with it, and two are separated from it by considerable intervals. As development 
proceeds the septa become constantly thinner and the intervening chambers increase 
correspondingly in size. This is effected partly through the increasing size of the larva 
and in part by a change of form in the entoderm cells covering the septa, which become 
far less elongated. Fig. 144 represents a section through the anterior part of a four days’ 
larva in which the radiating chambers have attained a considerable size. Fig. 145 is 
from the same specimen at the posterior end of the oesophagus; this corresponds very 
closely with the earlier stage shown in fig. 143. Fig. 146 is from the same specimen 
still further back, showing the free septa. The bilateral arrangement of the septa is 
strikingly shown in the symmetrical disposition of the septa of different widths 
(see p. 764). The entoderm cells have entirely changed their form, being now more or 
less flattened, or even forming in some places a flat pavement epithelium. On the 
edges of the septa have appeared the mesenterial filaments (ff.) but a description of 
these may conveniently be deferred to the following section. 
I have studied carefully the young septa for evidence of the participation of the 
ectoderm in their formation, but am led to conclude that they are formed almost 
exclusively from the entoderm, though in some cases a few ectoderm cells may make 
their way into the outer parts of the septa. In the youngest septa observed, the 
supporting lamella almost always appears as a simple membrane joining the lamella of 
the body-wall nearly at a right angle, and sometimes without interrupting its outline 
(fig. 147). In most cases, however, the lamella of the body-wall bends inwards 
