7 LARVAL DEVELOPMENT 59 
from the much narrower posterior part of the enteron, which 
grows out to become the oesophagus, stomach, and intestine. The 
notochord does not extend forward into the pharyngeal 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. 25,1). The 
mouth forms as a stomodaeum, or ectodermal invagination, 
antero-dorsally in the region where the neuropore has closed, and 
about the same time two lateral ectodermal involutions form 
(Fig. 26, A, 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. Ingrowths from 
the atrial pouches and outgrowths 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 exterior through the atrial apertures. Opinions differ 
as to whether only one or a few pairs of true gill-clefts are repre- 
sented in the young Ascidian; and the actual details of their 
formation and subdivision, to form the stigmata of the adult, 
differ considerably in different forms. In Clavelina the stigmata 
are formed as independent perforations of the pharyngeal wall: 
in Ascidia two pairs of protostigmata increase to six pairs, which 
are subdivided into stigmata; Botryllus and other forms are 
intermediate in some respects. No doubt the subdivision of proto- 
stigmata is primitive, but has been lost from the ontogeny in 
some cases. To what precise extent the walls of the atrial or 
peribranchial cavities are formed of ectoderm, or of endoderm, is 
still doubtful. 
The embryo is hatched about two or three days after fertilisa- 
tion, as a larva or Ascidian tadpole (Fig. 26, A) which leads « 
free-swimming existence for a short time, during which it develops 
its nervous system and cerebral sense-organs, and the powerful 
mesoblastic muscle-bands lying at the sides of the notochord 
(now a cylindrical rod of gelatinous nature surrounded by the 
remains of the original cells) in the tail which form the 
locomotory apparatus. Fie. 26, A, shows this stage, the highest 
in its chordate organisation, when the larva swims actively 
through the sea by vibrating its long tail with the dorsal and 
ventral fins. 
In addition to the structures already mentioned, the mesoderm 
