498 



TUNICATA. 



break up into segments corresponding to the buds. The neural tube, 

 genital strand and peribranchial and muscular tubes early break up 

 into such segments, but the entodermal and ectodermal parts of the 

 distinct buds remain connected together (Fig. 268, en) until a late 

 stage of development. 



We are still somewhat in the dark concerning the first origin of 

 these six longitudinal strands that appear in the body cavity. Accord- 

 ing to Seeliger, they are derived through simple differentiation from 

 the mesoderm-mass (Fig. 263, m) of the first stolon-rudiment. This 

 breaking up of the mesoderm into distinct longitudinal strands seems 

 to be determined to some extent by the form of the entoderm-tube, 

 which early assumes, in cross-section, a quadrangular form (Fig. 264), 

 its projecting edges, by approaching the ectoderm, causing the meso- 



Fig. 266. — Proliferating stolon of Salpa (after Todaro). b, blood-vessels of the stolon ; 

 ec, ectoderm ; en, entoderm-tube of the stolon ; es, endostyle of the parent (solitary 

 form) ; s, stolon. 



derm-mass to divide up into an upper, a lower and two lateral strands. 

 In this case, the neural tube and the peribranchial strands would 

 originate from the mesoderm, a derivation which from our knowledge 

 of the development of other Tunicates is not very probable. On this 

 point we refer the reader to our account of the budding of Pyrosoma 

 (p. 487). Other authors have attributed a different origin to the 

 peribranchial strands at any rate. These, according to Kowalevsky, 

 arise as independent growths from the atrial region of the parent, 

 while Salensky holds that they grow out of the pericardial vesicle 

 of the solitary individual. It is possible that Salensky's view was 

 founded on a confusion of these strands with Brooks' muscle-tubes. 

 In any case, it is possible that some of the longitudinal strands 

 of the stolon in Salpa are direct outgrowths of .he corresponding 

 systems of organs in the parent. An independent origin from the 



