PYKOSOMA DEVELOPMENT OF THE I'KOLI FERATTNG STOLON. 487 



The rudiment of the nerve-tube of the stolon also, according to 

 Seeliger, is derived from the genital strand. In very young stolons 

 (Fig. 256 A) the distal end of the latter appears to bend round to 

 the upper side. (The lower side of the stolon is marked by the 

 position of the genital strand g). This upper part of the genital 

 strand becomes, in the further course of development, separated 

 from the lower part, and, according to Seeliger, becomes the 

 rudiment of the neural tube (Fig. 256 C, n), a lumen very soon 

 appearing inside it. 



Seeliger thus holds that not only the genital organs of the bud 

 but also its peribranchial tubes, and a large part of all its mesodermal 

 structures, are derived from the genital strand. The group of cells, 



ff 





9 a ' is 



Fig. "2"iti. Three stages in the development of the proliferating stolon of Pyrosoma 

 (after Seeliger). In C, the separation of the two individuals (/ and II) is already 

 indicated, if, entoderm-process ; ec, ectoderm; es, endostyle of the parent ; g, 

 genital strand ; ks, first gill-slits ; ni, rudiment of the alimentary canal proper; n, 

 rudimenl oi the neural tube. 



which we described as the simple rudiment of the genital strand, has 

 thus, in his eyes, a far wider significance in the development of the 

 proliferating stolon. He has therefore designated it as the germ- 

 strand or the mesodermal germ-mass. 



The fact that Skeliger traces back the peribranchial tubes and the neural 

 tube in the buds of Pyrosoma to the mesoderm cannot fail to awaken surprise 

 when it is remembered that the rudiments of these organs are undoubtedly 

 derived from the ectoderm, not only in the Cyathozooid but also in the four 

 primary Ascidiozooids. in our attempts to interpret the structure and sig- 

 nificance of the proliferating stolon, we naturally seek for agreement between 

 the development of its buds and that of the first four Ascidiozooids. When 

 we consider that the proliferating stolon must evidently be traced back to the 



