CHAPTER V. 



THE TRANSFORMATION OF THE STOLON INTO THE SERIES OF 



AGGREGATED SALP^E. 



SECTION 1. Outline Sketch. 



The origin of the aggregated salpge is complicated by secondary 

 changes, but in its essential features it is a very simple history. I shall 

 therefore preface my account by a brief outline of the process, stated in 

 its simplest form, and divested of all secondary complications. 



I. The Proliferous Stolon. As already described, this, when fully 

 formed, consists, 1st, of a tubular sheath of ectoderm, Plate XXXIV, 

 Fig. 1, a, which is derived from the ectoderm of the solitary salpa; and 

 which contains, 2d, an endodermal tube, d', which is derived, as Plate 

 XXI, Fig. 1, d' shows, from the pharynx of the solitary salpa; 3d, a 

 nerve tube, Plate XXXIV, Fig. 1, Z, which is derived, as Plate XX, Fig. 

 4, I, shows, from the ectoderm of the stolon ; 4th, of two perithoracic 

 tubes, g and 7^, which are probably derived from the ectoderm of the 

 stolon, as shown in Plate XX, Fig. 3 ; 5th, an upper haemal tube, /, 

 which communicates at the base of the stolon with the cavity of the 

 heart of the solitary salpa, as is shown at j in Plate XXI, Fig. 3 ; 6th, 

 a lower ha3mal tube, i, which communicates with the body cavity of the 

 solitary salpa, as is shown in Plate XXI, Fig. 2, i ; and 7th, of a genital 

 string which consists of a series of eggs, m, inclosed in a follicular 

 sheath, n, and which is derived from the germinal mass, Plate XX, Fig. 

 6, n, of the solitary salpa. 



II. The Segmentation of the Stolon. The first indication of the seg- 

 mentation of the stolon is a series of ectodermal folds, Plate XXXIV, 

 Fig. 11, a, which first appear at its sides, but soon extend up and down 

 and completely encircle it, and, pushing inwards, mark out the body 

 cavities of the salpas, and also cut up the tubular structures inside the 

 stolon into segments. 



The active agent in this process of segmentation is the growth of the 

 ectodermal folds, and the other structures are actually cut by these folds, 



