Origin of the Organs of Salpa. 129 
lumen of the tube. ‘The presence of the ectodermal folds and 
the growth of the ganglia soon cause crowding and pressure, 
and the ganglion becomes flattened in the axis of the stolon 
and elongated transversely. As the oral ends of the pharyn- 
geal pouches grow up to the level of the ganglion, and push 
in between it and the ectoderm, the crowding becomes still 
greater and the single series of ganglia breaks up into two 
rows, which move to the right and left alternately as they 
grow, and the ganglion of a right-hand Salpa soon comes to 
le far away from that of the left-hand Salpa with which it 
was at first in contact. It is convenient to speak of this 
change as a “ migration ” of the ganglion ;_ but there is actually 
no migration, for the ganglion of each Salpa lies from the 
first in its final position on the middle line, dorsal to the oral 
end of the pharynx, and the apparent migration is the result 
of secondary changes in the position of the bodies of the 
Salpee, and is not due to any change in the relation of the 
ganglion to other organs of the body. 
Both Salensky and Seeliger have figured and described the 
“migration” of the ganglion; but ag they have failed to 
discover the rotation of the bodies of the Salpe, they regard 
it as an actual migration, and have completely misunderstood 
its true relation to the other organs of the body. 
My sections show that the “ subneural gland ”’ or ‘ ciliated 
funnel” is an outgrowth from the pharynx and that its inti- 
mate relation to the ganglion is secondary. Seeliger believes 
that the ganglionic rudiment gives rise to both the ganglion 
and the ciliated funnel, although he admits (p. 20) that his 
observations are not conclusive. 
The Ferithoracic Tubes and the Atrium or Cloaca of the 
Salpa Embryo.—It is not possible to describe the history of 
these structures intelligibly without figures. ‘They arise as 
involutions of the somatic layer of the follicle, and they attain 
to their final form before the blastomeres begin to replace the 
follicle cells ; so that there is a stage when the complete peri- 
thoracic system is outlined in cells which do not come from 
the fertilized egg, but from the follicle. 
This system makes its appearance, as it does in the embryos 
of ordinary ‘Tunicata, as a pair of lateral perithoracic invagi- 
nations, although in the Salpa embryo these are formed from 
the somatic layer of the follicle. They push inwards, pene- 
trate the visceral mass of follicle cells, and meet and unite on 
the middle line to form the median atrium or cloaca, From 
the level of the median atrium each perithoracic tube pushes 
dowuwards to the region where the cavity of the pharynx is 
subsequently to be hollowed out in the visceral mass. ‘The 
