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into closer relation with the neural gland, and one or more of the 
funnel-shaped ducts of the gland might, after having been carried in 
from the surface by the formation of the lateral atrial involutions, 
come to open into the ciliated depression of the tubercle in place of 
into the perihranchial cavity. The condition thus produced would 
he very much what Julin has described as existing in his specimens 
of Aseidia mammillata. If now for some reason the original openings 
into the perihranchial cavity became suppressed, leaving merely the 
secondary opening into the pharynx by means of the dorsal tubercle, 
we would arrive at the condition found in all ordinary Ascidians, 
It is not easy to see what the cause of the change could he, as there 
is no apparent advantage to he derived from it ; probably, however, 
there is no disadvantage since there is abundant communication 
between the branchial sac and the perihranchial cavity through the 
stigmata or slits in the wall of the former. 
This suggestion as to the origin of the present structure and 
relations of the neural gland aird neighbouring organs, in most 
Tunicata, implies that the pituitary body in the Vertehrata, which 
has lost its connection with the exterior, and probably also its 
firnction, has a similar history. In this view I am encouraged by 
some remarks by Balfour * from which it is clear that he considered 
the pituitary body, judging from its development, to have been 
originally a sense organ, opening into the mouth, and possibly 
correspondirrg to the Ascidian dorsal tubercle. He has also 
suggested f as an alternative the possibility that the neural gland in 
the Tunicata may he the homologue of the vertebrate pituitary body. 
This is of course the theory supported by van Beneden and Julin, 
and is open to the objection that it does not account for the 
remarkable structure of the dorsal tubercle. The view I hold com- 
bines both of those suggested by Balfour and Julin, by considering 
the pituitary body as the homologue of the neural gland, and as being 
therefore the rudiment of a primitive neural organ | which opened 
in the early Chordata by lateral ducts upon the side wall of the body; 
while the connection of the pituitary body with the stomodseum, 
* Treatise on Comparative Emlrtjology, vol. ii. p. 359, London, 1881. 
t Log. cit., p, 360, 
t Not the pronephros, since that is found along with the pituitary body 
in many Vertebrates, but possibly more ancestral. Might it not be the homo- 
logue of the provisional trochosphere excretory organs described by Hatschek 
and others in Polygordius and some Molhisca ? 
