402 LEFEVRE. [Vor. XIV. 
which represents a slightly older stage, leave little room for 
doubt that nuclei do wander out into the rudiment. But, on the 
other hand, I think that this figure and Fig. 27, c, show equally 
well that blood cells are added to the mass from the outside. 
I have, therefore, come to the conclusion that the ganglion 
has a double origin, and that the wall of the tube and free 
amoeboid cells coiperate in forming it. But, as we have seen 
that the dorsal tube ts made up of cells of the blood, tt ts, there- 
fore, to be remembered that the ganglion 1s ultimately derived 
solely from this source. 
The ganglionic rudiment is at first a very irregular heap of 
cells, and is closely associated at the periphery with surround- 
ing blood cells. The cell boundaries are completely lost very 
early, and the mass rapidly increases in size by multiplication 
of nuclei within, by further acquisition of cells from without, 
and by continued migration of nuclei from the wall of the tube 
(Fig. 27, c). The nuclei now arrange themselves in a couple of 
layers around a central core, in which fine fibrils are laid down, 
and the ganglion becomes completely marked off from the wall 
of the tube; the definitive structure is now attained. Fig. 27, 
d and ¢, illustrates the later course of development. 
My observations, therefore, have led me to believe that the 
hypophyseal tube and the ganglion are formed only in part from 
a common rudiment, and in this respect to take a middle ground 
between Hjort, Salensky, and Caullery, on the one hand, who 
have described a common origin for these structures in the 
ascidians studied by them, and Oka and Pizon, on the other, 
who maintain that they arise independently. In deriving the 
hypothesis and ganglion, however, from the cells of the blood, 
I differ widely from all previous observers, with the exception 
of Seeliger. Concerning the origin of the dorsal tube in the 
buds of Perophora, my results are totally opposed to the conclu- 
sion of Pizon, which, as stated above, is not based on sufficient 
evidence; namely, that “le tube dorsal des Pérophores a la 
méme évolution que le tube dorsal des Botryllidés” (/.c., p. 130). 
A study of the younger stages would have convinced him of his 
error. Pevrophora viridis, at all events, presents an exception 
to the general rule laid down by Pizon that “‘ Chez toutes ces 
