MUSEUM OF COMPARATIVE ZOOLOGY. 327 



26 of the present paper. This rod, crossing the blood current at 

 right angles, more effectually diverts this into the cup, so that a 

 steady stream now passes into and out of the latter, which rapidly 

 lengthens so as to form a tube projecting from the outer tunic into 

 the cellulose test (Fig. 27). 



The prolongation from the pericardium (Fig. 27, 2, and Fig. 7, 2) 

 also lengthens, and reaches almost to the tip or blind end of the tube, 

 and soon shows traces of a central longitudinal cavity (Fig. 27, 3), 

 which appears to be entirely closed at both ends. In a cross-section 

 at this stage we should have : first, the outer tube, thin-walled and 

 derived from the outer tunic ; within this a chamber continuous with 

 the sinus system, and within this a second tube, derived from the 

 pericardium, with very thick walls and a cavity without connection 

 with any of the pre-existing cavities of the embryo. This inner tube 

 now becomes flattened until its edges unite with the inner wall of the 

 outer tube, the cavity of which thus becomes " divided by a partition 

 into two canals, which are distinct for the whole length of the tube, 

 except at its very extremity, where they communicate just as the two 

 scalar of the cochlea do ; and it thence happens that, in the living 

 animal, a constant current passes up on one side of the partition and 

 down on the other, the direction of the two currents being generally, 

 but not always, reversed with the reversal of the general circulation" 

 (Huxley, p. 573).* 



* Huxley's figures of the early stages in the formation of the chain are very correct, 

 and exhibit the relations of the various parts correctly ; hut the passage above cpioted 

 includes nearly all of his description, which is correct as far as it goes, hut very brief. 

 Vogt also (Sur les Tuniciers nageants de la Mer de Nice, p. 36) gives a correct account 

 of the tube, but Leuckart, although he was acquainted with and refers to Huxley's 

 description, disputes its correctness. It is so easy to make observations at this period, 

 and their result is so satisfactory, that it is hard to understand how such a difference 

 of opinion could arise; but as Leuckart's statement is made with the greatest con- 

 fidence, it cannot be passed without notice, and is accordingly quoted here: "Man 

 hat behauptet, dass der rohrenformige Keimstock der Salpen aus mehreren ubereinan- 

 der gelegenen Hiiuten bestehe. Ich habe indessen — abgesehen natiirlich von der 

 iiussern Cellulosescheide, die sich bei der Entwickelung der Knospen in keinerlei Weise 

 betheiligt — vergeblich versucht, diese beiden Haute darzustellen. Das Keimrohr der 

 Salpen zeigt nur eine einzige Substanzlage, unci hat eine einfach-zellige Besehaffenheit. 

 .... Der Hohlraum, den die Keimrohre einschliest, communicirt, wie wir schon 

 fruker beschrieben haben, mit dem Lacunen-system des miitterlichen Leibes. Man 

 sieht auf das Deutlichste, wie die Blutku'rperchen an der einen Seitenwand der Keim- 

 rohre emporsteigen und spat&r an der entgegengesetzten Wand wiederum in den Kries- 

 lauf des miitterlichen Korpers zuriickkehren " (page 69). 



