ANATOMY AND DEVELOPMENT. 



205 



interior of the cells. The most anterior of the cells con- 

 taining the pigment is at first distinguished from the 

 others solely on account 

 of the fact that the pig- 

 ment-granules which it 

 contains are somewhat 

 larger than those in the 

 succeeding cells. (Cf. 

 Fig. 103.) 



Later on, however, 

 the first pigmented cell 

 is seen to separate itself 



enlo 

 Fig. 102. — Embryo of Ascidia mentula 



shortly before hatching ; from the right side. 



(After WiLLEY.) 



ck. Notochord, undergoing vacuohsation. 



(spinal cord). 0. Otocyst, lying on the floor 

 of the cerebral vesicle and projecting up 

 freely into its cavity. r.a. Right atrial involu- 

 tion. St. Stomodoeum. 



r .1 tu 1 •<- '• Eye. ent.c. Enteric cavity. / Adhesive 



from the others, and it p^pji,^^ „,^ interior portion of nerve-tube 



becomes gradually trans- 

 ferred by a differential 

 growth of the wall of 

 the vesicle down the right wall to its final position in the 

 ventral wall of the vesicle (Figs. 102, 103). This cell is 

 the otocyst, and the pigment-granules become consolidated 

 together to form the otolith. The latter is apparently 



Fig. 103. — Optical sections through cerebral vesicle of embryos of Ascidia 

 mentula, to show mode of origin of eye and otocyst. (After WiLLEY.) 

 e. Eye. o. Otocyst. 



extruded from the cell (otocyst) in which it was originally 

 formed, and the latter assumes a cup-shape, in the hollow 

 of which the otolith lies. The two structures together 

 form the so-called auditory organ, whose function may be 

 not so much of an auditory nature as that of an equilibrat- 

 ing apparatus. 



