486 



TUNICATA. 



- rns 



Fig. 254. — Transverse sec- 

 tion through the ento- 

 derm - process [en) of a 

 very young stolon-rudi- 

 ment of Pyrosoma (after 

 Seeliger). his, surround- 

 ing mesenchyme-cells. 



strand on the right is continued as far as the pericardial vesicle of 

 the parent. These strands are the so-called mesoderm-strands of the 

 proliferating stolon mentioned above, the development of which in 

 the four primary Ascidiozooids was followed by Salensky (No. 74). 

 The part played by these mesoderm-strands in the further develop- 

 ment of the proliferating stolon and of the 

 buds is still obscure, and therefore no 

 further note will be taken of it. 



That part of the ectoderm towards which 



the entoderm-proeess is directed (Fig. 253, 



ec) seems somewhat thickened even in the 



early stages. The actual stolon now begins 



to form in this region as a rapidly increasing 



swelling. The conical process that results 



(Fig. 256 A) is the stolon at the distal end 



of which transverse constrictions soon lead 



to the abstriction of individuals (Fig. 256 C). 



In transverse sections through a developing stolon (Fig. 255), the 



rudiments of the peribronchial tuhcs, (p) can already be seen at either 



side of the entoderm-proeess. How these rudiments arise is not yet 



accurately known, but as they are found connected by their distal 



ends in a certain way 

 with the genital 

 strand, Seeliger is 

 inclined to derive 

 them from the latter. 

 He therefore regards 

 the peribranchial 

 tubes in the buds of 

 Pyrosoma as meso- 

 dermal structures, 

 although, in the 

 Cvathozooid and in 

 the first four Ascidio- 

 zooids, they are un- 

 doubtedly derived 

 from the ectoderm. [Judging by what is known of the development 

 of these organs in other Tunicates, and of the relations of the genital 

 rudiment in all other animals, it appears to us in the highest degree 

 improbable that there is any connection between the two structures, 

 i.e., peribranchial tube and genital strand, or between the latter and 

 the nervous system]. 



— fi, 



Fig. 255. — Transverse section through two very young 

 stolons of Pyrosoma (after Seeliger). ec, ectoderm ; 

 el, elaeoblast-tissue of the parent ; en, entoderm- 

 proeess ; g, genital strand ; u, young egg-cell ; //. 

 peribranchial tubes. 



