ON THE DEVELOPMENT OF SAGITTA. 395 



passage it always kept exactly to the middle line, never 

 passing laterally into the cavity at the sides, and the drop of 

 water containing the carmine seemed to push apart the walls 

 enclosing the lumen of the gut, which was otherwise almost 

 or entirely obliterated by the walls coming into contact. 



The "septa" mentioned by Grassi seem, therefore, to be 

 the remains of the endodermal cells, and are not any way to 

 be regarded as mesodermal septa supporting the alimentary 

 canal. 



Papers refekked to. 



1. KowALEVSKY. — "Eiiibiyologischc Sludicii an Wiirnieni unci Ait.liio- 



poden," ' Mem. Acad. I'cteisbourg,' 7lli series, torn, xvi, 1871. 



2. BurscHLl. — " Zur Entwickluiigsgescliiclite der Sagilta," ' Zeilsclir. wiss. 



Zool.,' Bd. xxiii, 1873. 



3. Hertwig, O. — " Die Chaetognatlieii," ' Jeiiaische Zeit&clirift,' Bd. xiv, 



1880. 



4. GiiAssi. — " I Clietognati," ' Fauua und Flora des Golfes von Neapel, 



1883. 



5. JouKDAiN. — " Sur renibryogeiiie de Sagilta," ' Comptes read. Acad. 



Sci.,' Paris, 1892. 



EXPLANATION OF PLATES 19—21, 



Illustrating Mr. L. Doncaster's paper " On the Development 



of Sagitta." 



The figures of sections were drawn witli llie lielp of a camera hicida, using 

 a |r-incli or a jL.jnch objective. Tiie remaining figures are mostly drawn 

 from the living animal, and the camera lucida was not used. The niaguifica- 

 lions given are approximate. 



