ASCIDIA. 69 
The neural canai now differentiates into a tubular dorsal 
nervous system. The anterior end dilates to form the 
thin-walled cerebral vesicle (figs. 8 and 9, n.v.), containing 
later the intra-cerebral, dorsal, pigmented eye (oc.) and the 
ventral otolith (aw.) of the larva. The next part of the canal 
thickens to form the trunk ganglion, and behind that is 
the more slender spinal cord (7.c.), which runs to the extre- 
mity of the tail. A ciliated diverticulum of the anterior 
end of the enteric cavity (future pharynx), which enters 
into close relations with the front of the cerebral vesicle, 
and later opens into the ectodermic invagination which 
forms the mouth at that spot, is evidently the rudiment 
of the neural or hypophysial gland and canal. 
The future branchial sac (fig. 9, mes.), with.a ventral 
median thickening which will be the endostyle, is by this 
time clearly distinguishable, by its large size, from the 
much narrower posterior part of the enteron which grows 
out to become the cesophagus, stomach, and intestine. 
The notochord does not extend forward into the 
branchial region, but is confined to the posterior or 
caudal part of the embryo. It now shows lenticular 
pieces of a gelatinous intercellular substance secreted by 
the cells and lying between them (fig. 8). The mouth 
forms as a stomodzeum or ectodermal invagination antero- 
dorsally, in the region where the neuropore had closed 
up; and, about the same time, two lateral ectodermal 
involutions appear (fig. 9, at.) which become the atrial or 
peribranchial pouches, at first distinct, afterwards united 
in the mid-dorsal line to form the adult cloaca and atrial 
aperture. 
In-growths from the atrial pouches and out-growths from 
the wall of the pharynx coalesce to form the proto- 
stigmata (primary gill slits) by which the cavity of the 
branchial sac is first placed in communication with the 
