ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
775 
Julin, in asserting the presence of a communication between the nerve- 
tube and the stomod£eum. Before the cerebral vesicle of the Ascidian 
larva atrophies a portion of its wall becomes constricted off from the 
vesicle as a tube which opens in front into the mouth, while behind it is 
continuous with the rest of the neural canal. Yan Beneden and Julin 
are in error in supposing that the hypophysis is a thing apart from the 
neural tube ; they are at first one and the same, and it is only at a later 
period that the definitive hypophysis and the ganglion become differen- 
tiated and separated from one another. The ganglion of the Ascidians 
seems to bear the same relation to the hypophysis, that the infundibulum 
does to the hypophysis of the higher Vertebrates ; the difference is that 
in the latter these parts are rudimentary and arise independently, while 
in the former both organs are functional and arise together. 
Eyes of Salpse.* — Prof. 0. Biitschli reports that the simplest eye of 
a Salpa has the form of a single dorsal eye which forms a moderate 
projection ; it is chiefly composed of a retina, the cells of which are 
arranged radially to the surface. At the basal periphery there is a zone 
of pigment-cells which may certainly be said to have arisen by differen- 
tiation of the cells of the embryonic ganglion, just like the retinal cells. 
The latter, as in all eyes of Salpde , are composed of two kinds of cells, 
some of which are truly optic while others are supporting ; the latter are 
seen, in sections, to form a kind of network in the interspaces of which 
the optic cells are set. This simple eye receives its nerve-fibres directly 
from the brain, so that the free ends of the optic cells are turned towards 
the light. 
In other cases there is a certain differentiation of the eye, for the 
lateral parts of the retina begin to press outwards, and their nervous 
supply becomes somewhat altered. This differentiation of the primi- 
tively simple eye into three parts, a median and two lateral, may be very 
well seen in those forms in which the eye is converted into a horse-shoe- 
shaped ridge, open anteriorly, and placed on the dorsal side of the ganglion. 
The median part has the same minute structure as in the already 
described simple eye ; but the retinal cells of the lateral parts tend more 
and more to take up a position perpendicular to that which they 
originally held. Their nerve-fibres are, consequently, distorted and form 
a plexus, and their pigment now occupies the outer end of the retinal 
cells. As a result of this, the median part of the eye has the ordinary 
character of a non-inverted eye, while the lateral parts have become 
inverted parts. This is very well seen in other Salpae i where the three 
parts have become sharply differentiated from one another, and where 
they form three eyes on the anterior end of the brain. 
The author justly remarks that this change is remarkable, and the 
fact has certainly a deep morphological and phylogenetic significance. 
It is difficult to compare the simple Saipan eye with the pineal eye of 
Vertebrates, unless we suppose that it was covered by a very delicate 
membrane, which was continued peripherally into the zone of pigment- 
cells. If we may suppose that this eye was primitively vesicular 
we could understand the connection between it and the vesicular 
eyes of Vertebrates, where the brain itself remains hollow. The 
* Zool. Anzeig., xv. (1892) pp. 349-53 (5 figs.). 
