Origin ofthe Organs q/Salpa. 129 



lumen of tlie 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 

 lie 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 as they have failed to 

 discover the rotation of the bodies of the Salpa3, 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 tlie complete peri- 

 thoracic system is outlined in cells which do not come from 

 the fertilized ^^^^ 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 Irom 

 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 

 downwards to the region where the cavity of the pharynx is 

 subsequently to be hollowed out in the visceral mass. The 



