REPORT ON THE THNICATA. 
91 
The next process revealed by the sections is the longitudinal division of the arch- 
enteron into three cavities by the formation of two laterally placed lines of constriction. 
These cut across the cavity and unite with the opposite wall, separating off two lateral 
thin-walled spaces from the central much stronger sac (PI. IX. fig. 7 ; PI. VIII. fig. 8). 
TVhile this division has been taking place, or possibly before the process began, the 
neural tube has been formed. Unfortunately none of the specimens throw any light on 
its origin. This I more especially regret, as from Kowalevsky’s observations it appears 
that in Didemnium styliferuin and Amaroiicium proliferum the neural tube is derived 
from the dorsal waU of the central compartment or mesenteron, an origin which, on 
theoretical grounds, seems very improbable. It is first observed (in my transverse 
sections) as a small but thick-walled tube, lying close under the ectoderm in the dorsal 
region, formed of a single layer of wedge-shaped nucleated cells and having a very 
small lumen (PI. YIII. fig. 8). I am inclined to believe (although I have no specimens 
which prove it) that the neural tube arises, as in the development of the Ascidian from 
the egg, by the formation of longitudinal dorsal ridges of ectoderm which grow up, arch 
over, and coalesce, thus forming an ectodermal canal which occupies the dorsal region 
of the perivisceral cavity (the space between the primitive ectoderm and endoderm, not 
the future peribranchial cavity). 
In sections where the archenteron is completely divided into three cavities the 
neural canal is always seen. It afterwards increases in size and the lumen enlarges ; it 
also separates farther from the ectoderm and takes up a position nearly equidistant from 
the two primitive layers, or in some cases nearer to the endoderm (PI. IX. fig. 8, n.t.). 
The bud as a whole has now grown considerably, and the ectoderm has changed its 
appearance. The number of cells is much greater, and individually they are longer, 
having a distinctly columnar form and a yellowish colour. 
The three cavities into which the archenteron was divided also continue to increase 
in size, especially the central one, which is destined to form the alimentary canal, and 
chiefly the branchial sac. The two lateral cavities are not nearly so large, and their 
walls are composed of smaller cells. They tend to apply themselves to the outer wall of 
the central cavity and to curve round its ventral and dorsal edges; eventually they unite 
at the latter and form the peribranchial cavity or atrium. 
An important process may be seen taking place in the ventral region of the primitive 
branchial sac. Soon after the division of the archenteron, two longitudinal lateral ridges 
grow inwards along the ventral part of the central division, leaving a deep groove between 
them. This is the origin of the endostyle. At one time it occupies nearly the whole 
breadth of the ventral region of the future branchial sac (PI. IX. fig. 8, en.), and the 
cells of which it is composed are considerably larger than those of the rest of the wall. 
Two buds which were cut in longitudinal section show (PI. IX. figs. 9, 10) a 
further stage of development, in which thi-ee points of interest are seen, viz., (1) the 
