GIBSON— CEPHALOCHORDA : " AMPIIIOXIDES." 231 



developed. The lumen of the tubule is largest in the last of the series, and decreases in 

 size as we pass forwards. 



Goldschmidt's theoretical interpretation of the gill-slits of Amphioxides would rather 

 lead us to expect a double series of nephridia symmetrically disposed on either side of 

 tliem. This is emphatically not the case. Bat 1 do find what may possibly be the 

 beginnings of a second series, in an exactly similar position — far up on the right wall of 

 the pharynx — to that in which they appear, in relation to the right-hand series of 

 gill-slits, in the Ampldoxtis larva {vide infra). In sections through the middle region 

 of the pharynx thei-e are seen at regular intervals, rather longer than those separating 

 the gill-slits, little aggregations of cells occupying the end of the ccelomic diverticulum 

 included by the right lateral pharyngeal fold. The nature of these aggregations can 

 only satisfactorily be seen anteriorly ; here, in the region of the hinder end of the mouth, 

 we can see, at rather more frequent intervals, small septa springing from the lateral 

 wall (separating splauchnocoele and myocoele) at their anterior end, and passing back 

 obliquely onto the dorsal splanchnopleure lining the roof of the coelomic diverticulum 

 included in the fold, to end at the angle of the latter; at the point where it ends 

 ])osteriovly, each septum increases in breadth and shuts off between itself and the 

 pharyngeal wall a small, backwardly directed pouch of the splanchnocoele {vide PL 15. 

 lig. 12). The first of these septa lies opposite the 5th gill-slit, and ends apposed to the 

 dorsal arm of the club-shaped gland ; behind the 8th (corresponding to the 11th gill-slit) 

 the lateral portions of the septum do not occur, and each is represented solely by the 

 cffilomic pouch, the true nature of which is often impossible to make out, owing to the 

 confined space in which it lies ; the 21st and last of these pouches occurs opposite the 26tli 

 gill-slit. It should be mentioned that the coelomic epithelium on both sides of the 

 septa, where they underlie the pouches, is of a more cubical character than that found 

 elsewhere. 



In order to elucidate these appearances, I must refer to the extremely valuable paper 

 on the development of the right series of gill-slits and excretory tubules in the 

 Amphioxus larva, now in the press and shortly to appear in the ' Anatomischer Anzeiger ' 

 ( 1909), which, by the kindness of the author, I have been allowed to read ia manuscript. 

 Ey this, the first account we have of the subject, the mesodermal origin of the tubules — 

 at any rate of tlie right-hand series — seems to be established beyond all doubt; for they 

 are shown to be formed, before any fusion of atrial and pharyngeal walls exists in their 

 neighbourhood, as splanchnopleuric funnels growing downwards between branchial and 

 coelomic epithelia immediately dorsal to the points where, a little later, the secondary 

 slits will break through. Of great importance for our interpretation of Amphioxides is 

 the presence, in the Amphioxus larva also, of inter-metameric septa very similar to those 

 which I have just described; posteriorly these are somewhat displaced from their 

 original position, but " au niveau de la l*"'" poche, les insertions internes se prolongent 

 vers le has et I'arri^re sur I'ebauche du canalicule de Boveri. La conclusion qui se 

 degage de ce rudiment de segmentation metamerique du splanchnocoele serait que le 

 jeune canalicule de Boveri s'ouvre par son orifice coelomique dans le segment situe 

 devant lui." 



31* 



