ASCIDIACEA -DEVELOPMENT OF THE FREE-SWIMMING LARVA. 359 



the ventral wall of the vesicle while the free end carries a pigment 

 cap. This view of the formation of the auditory organ is supported 

 by KUPFFER'S statement (No. 35) that the cells of the ventral wall 

 of the sensory vesicle which surround the otolith-cell and which 

 differ somewhat in histological character from the rest, are provided 

 with fine, stiff setae projecting towards the otolith-cell. According 

 to KUPFFER, a vesicular cavity is found in the crista acustica 

 directly below the otolith-cell. 



According to KOWALEVSKY, the otolith-cell, on its first appearance, is 

 situated on the dorsal wall of the sensory vesicle and only shifts later over 

 its right side to the ventral surface. Our knowledge of the structure and 

 development of both the sensory organs is, however, very inadequate.* 



The frtnik-xfrfittn of the nervous system (Figs. 167 and 168, r) 

 (the trunk-ganglion) is, according to VAN BENEDEN and JULIN, 

 the direct continuation backward of the left dorsal swelling of the 

 sensory vesicle. The cells of the wall of this swelling show the same 

 histological character as those which, in a single layer, line, like an 

 epithelium, the narrow central canal of the trunk-ganglion. On 

 these cells, however, on the ventral side, there is superimposed a 

 great mass of ganglion-cells (Fig. 171). According to the distri- 

 bution of these cells we can recognise the division of the trunk- 

 ( i>-anglin into an anterior and a posterior section, the anterior section 

 being still included by KUPFFER in the cerebral sensory vesicle as 

 a ganglionic portion. In the posterior section, the ganglion-cells 

 enclose a nucleus of nerve-fibrillae. The trunk-region of the nerve- 

 <())( I lies above the most anterior end of the chorda (Fig. 168 A) 



* [According to WILLEY (No. XXXVI.) who has investigated the develop- 

 ment of these organs in Ascidia mcntula and Clavelina lepadiformis, the first 

 indication of these sensory organs consists in the deposition of black pigment- 

 granules in the dorsal wall of the cerebral vesicle. The most anterior of the 

 cells containing pigment-granules becomes distinguished by the larger size 

 of its granules and the swollen nature of the cell itself. This pigment-cell 

 soon separates itself from the others and becomes gradually transferred by 

 a differential growth of the wall of the vesicle down the right wall to its 

 final position on the ventral side of the vesicle. This cell is the otocyst, and 

 the pigment-granules become consolidated together to form the otolith. The 

 other pigment-cells of the dorsal wall of the vesicle collect themselves 

 together and form a slight protuberance in the right dorso-lateral corner 

 of the vesicle. The pigment-granules become concentrated toward the cavity 

 of the vesicle. Subsequently two or three cells from the adjoining wall of 

 the vesicle take up a position, one above the other, in front of the mass of 

 pigment and by an alteration of their contents give rise to the lens of the 

 eye. The original pigment-producing cells constitute the retina, which retains 

 its primitive position as part of the epithelial wall of the brain. See also 

 SALKNSKY (No. XXX.). ED.] 



