ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
51 
the outer wall is differentiated into retinal and lens cells. The trunk 
part of the cord persists as the visceral nerve ; the rest degenerates. 
In the ventral individual of Diplosoma , the secondary funnel and the 
cerebral ganglion arise from the lateral neural canal, the other parts 
(sensory vesicle and cord) are wanting. 
An epithelial enteric plate arises from the endoderm, two lateral 
protrusions form the lateral sacs, which grow axially and fuse to form 
the axial enteric cavity. From the latter is formed the branchial sac 
in Didemnum , in Diplosoma only that of the dorsal individual. The 
lateral sacs are in part the rudiments of the intestinal canal, in part of 
the branchial sacs. 
In Diplosoma, the right lateral sac divides into an anterior 
branchial sac (rudiment of the branchial sac of the ventral individual) 
and a posterior stomach sac, which forms the stomachs of both indi- 
viduals. The left lateral sac — the rudiment of the intestines of both 
individuals — remains undivided. In Didemnum , a caecum arises at the 
boundary of the right lateral sac and the axial enteric cavity ; this is 
the right branchial sac. The right lateral sac becomes the stomach. 
A left branchial sac also arises in front of the left lateral sac. 
The two branchial sacs of Didemnum approach ventrally, unite with 
their blind ends and form a vesicle — the pericardium. The stomach 
sac unites ventrally with the intestinal sac ; the latter is freed from the 
branchial sac, unites with the left peribranchial sac, and opens finally 
through the anus into the cloacal cavity. In Diplosoma the matter is 
much more complicated. 
The peribranchial sacs arise as ectodermic invaginations, a pair in 
Didemnum , two pairs in Diplosoma. 
Two lateral evaginations from the branchial sac of Didemnum grow 
in on the peribranchial sacs, fuse with them, and in breaking into their 
cavity form the two j)rimary clefts. The others are much later in 
appearing. The dorsal parts of the peribranchial sacs grow towards 
one another and fuse to form the cloacal cavity, into which a cloacal 
invagination opens from the outside. 
The cellulose mantle is formed at the expense of the kalymmocytes, 
but a fibrous layer in Didemnum is due to processes of ectoderm cells. 
Even before hatching the embryos begin to bud. Numerous small 
evaginations arise from the oesophagus, and during metamorphosis (later 
in Didemnum ) these form new individuals. In Diplosoma the buds 
appear on both individuals. 
B. The Organogenesis of Tunicata. — From this chapter we can only 
state some of the general conclusions. 
(1) As to the nervous system. — The primary form is that of Appen- 
dicularian and Ascidian larvae, consisting of primary funnel, sensory 
vesicle, and the nerve-cord. The cerebral ganglion of Tunicates is 
secondary, and arises from the wall of the sensory vesicle or from the 
posterior wall of the primary funnel. In the Pyrosoma and Salpa types 
the nervous system corresponds solely to the primary funnel. Except 
in the Appendicularia type the primary form of sense-organ — the sensory 
vesicle — is either only embryonic or absent. All other kinds of optic 
and auditory organ are of secondary origin. 
(2) The mesoderm. — There is no substantial evidence of primary 
metamerism or of a coelom. The mesoderm arises from two plates 
E 2 
