Origin of the Organs of Salpa. 13s 
ryngeal pouch. While the vesicles are hollow from the first, 
they have at first no communication with the cavities of the 
pharyngeal pouches. The first trace of the gill-slit is a fold 
or diverticulum in the dorsal wall of the pharyngeal pouch, 
This elongates and soon unites with the wall of the peri- 
thoracic vesicle to form a gill-slit. Soon after these are 
formed the posterior ends of the bodies of the Salpe begin 
to push out to the right and left im such a way that the ellip- 
tical cross section of the body becomes converted into a wedge, 
with its narrow edge on the left side of a right-hand Salpa, 
and on the right side of a left-hand Salpa. The two peri- 
thoracic vesicles are differently affected by this change, for 
while the one nearest the pointed end of the wedge is com- 
pressed in the line of the axis of the stolon, the other is not. 
‘Thus the left perithoracic vesicle of a right-hand Salpa and 
the right one of a left-hand Salpa become flattened and elon- 
gated towards the middle line, while the other remains more 
nearly circular in section. Their relations to the morpho- 
logical middle plane are fundamentally identical, but as the 
middle plane itself gradually moves outwards there is an 
apparent asymmetry. 
Each perithoracie vesicle now becomes extended towards 
the middle line, where they unite to form the median atrium 
or cloaca, to which they contribute equally, although the 
position of the body is such that sections transverse to the 
long axis of the stolon might easily be misinterpreted and 
held to prove that the whole median atrium of a right-hand 
Salpa arises from the left vesicle alone, and that of a left-hand 
Salpa from the right one alone. ‘The secondary changes of 
position are, however, of such a character that it is impossible 
to describe them in detail without figures. 
Seeliger’s account of the perithoracic structures of Salpa 
democratica (pp. 18, 48, and 63) serves to show how difficult 
the study of a simple structure may be made by a slight 
change of position, for phenomena which can be observed with 
ease in the straight stolon of Salpa pinnata are so obscure in 
Salpa democratica that all the industry and technical skill 
which Seeliger has devoted to this species has had very little 
outcome. 
His account of the history of the perithoracie system is 
essentially as follows :—The perithoracic tubes, which he calls 
the ‘‘ Seitenstriinge,” are mesodermal in their origin, and are 
specialized out of a mass of mesoderm cells which gives rise 
also to the nerve-tube of the stolon and to the genital rod. 
The mesoderm passes into the stolon from the body of the 
embryo in an unspecialized condition, and gradually becomes 
