134 Mr. W. K. Brooks on the 
differentiated into these organs after the stolon is formed. 
The folds in the ectoderm of the stolon divide the ‘‘ Seiten- 
striinge’”’ into a series of solid masses at the sides of the 
stolon between the ectoderm and the endoderm. These bodies 
are equal in number to the future Salpx, and not twice as 
numerous. As each Salpa is constricted off from the tube it 
carries with it the greater part of one of these masses of cells 
from one side of the stolon and the lesser portion of the one 
on the opposite side. These two masses are not bilaterally 
placed in the body, but are on the middle line, the larger one 
being dorsal or neural and the smaller one ventral or hemal. 
The latter gives rise to the heart and to the eleoblast, while 
the larger one, on the neural surface, gives rise to most of the 
mesoderm of the chain Salpa and also to a cloacal vesicle 
which is median and unpaired. 
The vesicle becomes distended, and at two points, one on 
each side of the middle line, it unites with the wall of the 
branchial sac, and the cloaca and the branchial chamber thus 
become connected through the two gill-slits, while a similar 
union with the ectoderm in the middle dorsal line forms the 
cloacal aperture. Seeliger’s account is perhaps as near the 
truth as one could hope to get by the study of transverse 
sections of the twisted stolon of Salpa democratica; but a 
very little study of sections in other planes in more favourable 
species will show that he has completely failed to understand 
the subject and that his account has no permanent value. 
It is not only irreconcilable with my own observations, but 
also with our knowledge of Pyrosoma, for both Seeliger 
(‘Pyrosoma,’ pp. 622-624) and Salensky (‘ Pyrosoma,’ pp. 31— 
386) state that in this genus the perithoracic system is bilate- 
rally symmetrical, that each bud has two perithoracic vesicles, 
which are not dorsal and ventral, but right and left, that each 
of them unites with its own side of the pharynx to form the 
gill-slits before the two vesicles unite with each other to form 
the median atrium, and that this arises, as it does in the 
ageregated Salpa, on the dorsal middle line by the meeting 
and union of diverticula from the two vesicles, and that the 
external aperture arises still later, as it does in Salpa, as an 
independent aperture on the middle line. 
The perithoracic vesicles are derived, as I find that they 
are in Salpa, from the right and left perithoracic tubes of the 
stolon; but, in the primary ascidiozooids at least, these are 
continuous with the perithoracic tubes of the primary embryo 
or cynthozooid, where, according to both Kowalevsky and 
Salensky (pp. 466, 473-475), the evidence that they arise as 
