502 



TUNICATA. 



towards the proximal end of the stolon is now turned toward the outer 

 side of that structure (Fig. 269), while the ventral side has passed from 

 its original distal position and is turned toward the median line of the 

 stolon. This rotation, which was pointed out by BROOKS, does not 

 take place simultaneously throughout the whole of the bud, but 

 affects the posterior or ab-oral end at which the elaeoblast lies earlier 



than the anterior or oral end. 

 If, therefore, we carefully examine 

 a somewhat oblique longitudinal 

 section through the stolon, like 

 that given in Fig. 270, we shall 

 be able to follow the process of 

 shifting, since the individuals are 

 in consequence cut through at 

 different planes. We find that 

 individuals 16-21, the posterior 

 ends of which are cut through, 

 are already in their final position, 

 their arrangement being distinctly 

 biserial. The individuals of the 

 left row (16, 18, 20) have their 

 dorsal side directed to the left, but 

 the ventral side turned to the 

 middle line of the whole stolon. 

 The individuals of the right row 

 show an opposite arrangement. 

 When, however, we examine the 

 buds the anterior part of which 



FIG. 271. Diagram illustrating the 

 .relative position of the individuals 3-7 

 depicted in Fig. 270 (after BROOKS). 

 D, distal ; P, proximal ; R, right ; L, 

 left (with regard to the stolon), en, 

 entoderm - tube of the stolon ; hs, 

 haemal side of the bud ; I, left side of 

 the bud ; Ib, left half of the branchial 

 sac ; n, neural side of the bud ; o, upper 

 blood-sinus in the stolon ; r, right side 

 of the bud; rb, right half of the 



is cut through (7-11), we find 

 that th>> biserial arrangement is 

 here not fully carried out, the 

 individuals of the left row lying 

 partly on the right and those of 



t b he U stolmi SaC; M ' lowerb] - sinusin the right row partly on the left 



of the stolon. The rotation also 



is not complete. The endostyle (es in individuals 8) which marked 

 the ventral (haemal) side, is still directed distally (D). The neural 

 side, which is marked by the position of the ganglion (n in individual 

 11), still to a certain extent retains its primary position, i.e., is still 

 turned towards the proximal end (P) of the stolon. We here see 

 very clearly the way in which the final biserial arrangement of 



