292 JOHNS HOPKINS UNIVEESITY MOKPHOLOGICAL MONOGRAPHS. 



to lie far below the level of the stolon. For a long time the ganglion 

 retains its proximal position in Salpa pinnata, Plate VI, Fig. 4, s, and 

 Plate XXXIII, Fig. T, s, and this position is retained for a still longer 

 time in Salpa cylindrica, Plates VIII, Fig. 2, s, and XL, Fig. 14, s and 

 19, s, but it ultimately moves outwards away from the axis of the chain, 

 as Plate VIII, Fig. 1 and Plate XXXVIII, Fig. 41, show. I must repeat 

 that there is no real migration of the ganglion ; that it occupies its true 

 position with reference to the body of the salpa at all stages from the 

 first, Plate V, Fig. 1, to the end, and that its apparent migration is due to 

 changes in the position of the body of the salpa. At all stages it lies in 

 the middle plane, dorsal to the pharynx, at the end of the body, and thus 

 furnishes a conspicuous index to the changes in the position of the 

 middle plane. 



Seeliger has correctly described the segmentation of the nerve tube 

 and the "migration" of the ganglion, which he erroneously regards as 

 an actual change in its position with reference to the other organs of the 

 body. He also believes that the ganglionic rudiment gives rise, not only 

 to the ganglion, but to the ciliated groove or sub-neural gland, although 

 he admits that his observations upon this latter point are not conclusive 

 (11), page 70. My own observations show clearly that the ciliated groove 

 is not, in origin, part of the ganglion, but an independent outgrowth 

 from the roof of the pharynx. It is shown at a young stage when it ends 

 blindly, and has no connection with the ganglion in Plate VI, Fig. 4 ; 

 Plate VII, Fig. 2, and in Plate XXXVI, Fig. 10, v, and Fig. 12, v. 



SECTION 8. The Perithoracic Tubes and the Atrium, or Cloaca of 



Salpa Embryo. 



The history of the perithoracic structures of the salpa embryo has 

 been described on pp. 29-35 and on pp. 123 and 124, and I have shown 

 that while they are completely outlined in follicle cells before their 

 blastodermic epithelium is acquired, the course of their development is 

 intelligible only on the hypothesis that they are homologous 'with the 

 corresponding structures of other tunicates. 



Salensky has described the origin of the "gill" and of the median 

 atrium or cloaca of the salpa embryo in a number of species, but the 

 reader of his papers may search in vain for any basis of comparison 



