70 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
exterior through the atrial apertures. Opinions differ as to 
whether only one or a few pairs of true gill clefts are 
represented in the young Ascidian, and the actual details 
of their formation and sub-division differ greatly in differ- 
ent forms. ‘To what precise extent the walls of the atrial 
or peribranchial cavities are formed of ectoderm or endo- 
derm, is also still doubtful. 
The embryo is hatched, about two or three days after 
fertilisation, as a larva or Ascidian tadpole (fig..9) which 
leads a 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, and forming the locomotory 
apparatus. Figure 9 shows this stage, the highest in its 
chordate organisation, when the larva swims actively 
through the sea by vibrating its long tail provided with 
dorsal and ventral fins. 
In addition to the structures already mentioned, the 
mesoderm has formed the beginning of the muscular body- 
wall and the connective tissue around the organs, and has 
elven rise to the blood, the endostyle has developed as a 
thick-walled groove along the ventral edge of the pharynx, 
which now becomes the branchial sac, and the pericardial 
sac and its invagination the heart have formed in the 
mesoblast between the endostyle and stomach. ‘The 
unpaired optic organ in the cerebral vesicle, when fully 
formed, has a retina, pigment layer, lens and cornea; while 
the ventral median sense-organ is a large spherical, partially 
pigmented otolith, supported by delicate hair-like processes 
on the summit of a hollow ‘‘crista acustica”’ (fig. 9). 
Both the otolith and the retina and lens of the eye are 
formed originally by the differentiation of a group of cells 
