764 DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
slightly at the point where the septum meets it, and ectoderm cells with conspicuous 
nuclei may sometimes be seen lying directly in the angle thus formed (fig. 148.) (This 
figure is from the peduncular septum, but answers equally well for the radial septa.) 
In a very few cases the lamella appears to be actually infolded to some extent at the 
base of the septum, and ectoderm cells pass into the space thus formed, and thus come 
to lie within the body of the septum. In still other cases this fold appears to close 
up, forming a small triangular space at the root of the septum in which one or two 
ectoderm cells appear, as shown in fig. 149, n. These never extend far out into the 
septum, however, and the greater portion of the lamella of the latter is secreted, as 
I believe, by the bases of the entoderm cells. 
The question as to whether the lamella of the septum is double, and contains 
ectoderm cells invaginated from the exterior, is one of much theoretical interest, since, 
if this be the case, the septa are to be regarded as actual infoldings of the entire body- 
wall, and not as simple entodermic ridges. Lacaze-Duruigers in his beautiful 
memoirs on the development of polyps,* expressly states that both of the layers of the 
body-wall participate in the formation of the septa, and he figures in the larvee of 
Astroides calycularis ectoderm cells with numerous nematocysts passing directly into 
the body of the septum. On the other hand, he is strenuously opposed by 
KowWALEVskKY, who maintains that the entoderm alone is concerned in the formation 
of the septum. My own observations throw no new light on this interesting question ; 
for although the great bulk of the septum with its lamella is in Renilla certainly 
entodermic, yet the occasional entrance of a few ectoderm cells into the base of the 
septum may indicate that an invagination of ectoderm originally occurred, in connexion 
with a special development of the underlying entoderm, but was subsequently nearly 
or completely lost. The matter is certainly worth further investigation in other polyps, 
for it is difficult to believe that Lacaze-Duruigrs’s figures rest upon no other basis 
than pure imagination. 
b. Arrangement of the septa. 
The septa are grouped about the cesophagus with a definite relation to the dorso- 
ventral axis, as shown in transverse sections (fig. 142). The cavity of the cesophagus 
is elongated in the dorso-ventral axis, and its angles are opposite two compartments, 
which may in KOLLIKER’s terminology be called the dorsal and ventral chambers. On 
each side of the cesophagus are, therefore, three chambers which are called respectively 
the dorso-lateral, median lateral or simply lateral, and ventro-lateral chambers. 
Following the same terminology, the septa may be designated as dorsal, dorso- 
lateral, ventro-lateral and ventral, respectively, there being four on each side of the 
cesophagus. 
This bilateral grouping of the septa becomes very conspicuous in transverse sections 
* arch. de Zool. Exp. et Géner, tome i., il. 
