of Edinh'urgli, Session 1882-83. 
149 
into wliicli the rectum and the genital ducts open. Its lining 
membrane is derived from the epiblast, being formed in the embrjm 
by a pair of lateral involutions which afterwards fuse dorsally. 
In two specimens of Ascidia mammillata which I had an 
opportunity of examining recently, I found the neural gland in 
precisely the condition described by Jnlin, but its duct had no 
aperture into the pharynx, the dorsal tubercle being entirely absent. 
The small funnel-shaped apertures into the peribranchial cavity were 
numerous and well developed, so that in the case of these individuals 
the neural gland was connected with the cloacal part of the peri- 
branchial cavity only, exactly the arrangement to be expected if 
the gland had a renal function. Admitting that that is so, and 
that the neural gland is the homologne of the hypophysis, it seems 
possible to me that this, or something like this, may have been the 
condition of affairs in the primitive Chordata previous to the point 
of divergence of the Urochorda.* There may have been a renal 
gland placed ventrally to the nervous system — not necessarily at the 
anterior end only, — and opening on the surface of the bodyt by 
one or more laterally placed apertures — this gland being represented 
in the Tunicata by the neural gland, and in the Vertebrata by the 
glandular portion of the pituitary body. 
As to the dorsal tubercle, whether it has now any sensory function 
is doubtful, on account of the apparent absence of nerve-supply ; 
but I consider that it must have been a sense organ formerly — 
possibly placed at first on the surface of the body close to the mouth 
(the branchial aperture), since the anterior part of the pharynx 
develops from the epiblast as a stomodaeum, — and I would suggest 
that the connection of the tubercle with the duct of the neural gland 
may be an afterchange, caused possibly by the enlargement of the 
pharynx into a branchial sac, and the development of the peri- 
branchial chamber. It may readily be imagined how, as the result 
of the formation of these cavities, the dorsal tubercle would be brought 
I have adopted Balfour’s classification {Comparatim Embryology, vol. ii. 
1881) as follows : — 
Chordata. 
1. Urochorda (Tunicata). 
2. Cephalochorda {Amphioxus). 
3. Yertebrata. 
t The lining of the peribranchial cavity into which the ducts open in 
Ascidia mammillata has been already shown to be continuous with the 
ectoderm on the surface of the body. 
