^ S,S TUNICATA. 



remains of the duct connecting each zooid with its neighbours (Fig. 196, s 

 p. 402 ; Fig. 253 ; v), and when we remember that in the four primary Ascid'io- 

 zooids continuations of the peribranchial tubes also are originally contained 

 in these ducts, we shall be led to ask whether, after the abstriction of those 

 parts of the peribranchial tubes that belong to the separate Ascidiozooids from 

 the ducts, remains of the tubes may not be retained in the form of incon- 

 spicuous cell-groups which might easily be mistaken for mesoderm, and from 

 which the peribranchial tubes of the stolon that develops later are derived 

 The rudiment of all the peribranchial tubes which .form later could in this 

 way be traced back to the peribranchial tubes in the Cvathozooid. Indeed 

 an extension of this way of viewing the matter might perhaps lead us to con- 

 jecture that the explanation of the many conflicting statements as to the rise 

 of the primary organs in the proliferating stolon in the various groups of 

 Tumcates is perhaps to be found in the theory that no primary organs originate 

 afresh m the proliferating stolon, but that they can all be traced back to corre- 

 sponding organs in the embryo from which thru become abstricted. Such a 

 conjecture receives special support from the condition of the ventral stolon 

 iu Doliolum. 



We might well feel inclined to find some genetic relation between the peri- 

 branchial tubes of the Pyrosoma bud and the so-called mesoderm-strands of 

 the stolon mentioned above. After Salensky's statements as to the rise of 

 these strands (No. 74), such an assumption seems to us improbable, and we 

 prefer to derive the mesodermal structures of the bud from these strands. 



When the stolon is perfectly developed, the typical aspect presented 

 by it in cross-section (which recurs also in the Salpidae) is that de- 

 picted in Fig. 257. The entoderm-tube (en) is cruciform, two limbs 

 «>f the cross extending upwards and two downwards ; the fold between 

 the former gives rise to the endostyle, while the two latter may already 

 be considered as the rudiment of the stomach and intestine. In fch_ 

 spaces between the four outgrowths of the entoderm-tube are four 

 strands, three of which already possess lumina. The dorsal strand 

 (n) is the nerve-strand, the lateral strands are the peribranchial 

 tubes (p), and the ventral space is occupied by the genital strand (g). 

 In later stages, by the development of the primary body-cavity, these 

 strands become separated from the entoderm-tube; the mesoderm 

 (nu), at the same time, appears both as isolated mesenchyme-cells 

 and as cell-masses which, according to Seeligek, must to some 

 extent be traced back to the genital strand (Fig 255 B, g). 



The breaking up of the stolon into separate individuals takes place 

 by means of constrictions (Fig. 25G C). Through the development 

 of these, the peribranchial tubes, the neural tube and the genital 

 strand of each individual become completely separated from the 

 corresponding rudiments of the adjacent buds. The enteric tube 

 retains its continuity longer; even in highly developed buds we find 



s 



e 



