ANATOMY AND DEVELOPMENT. 



205 



nt 



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, 

 the first 

 is seen to separate itself 



however, 



Fig . I02 _ Embryo of Ascidia mentula 



Cell snort 'y before hatching; from the right side. 

 (After WILLEY.) 



ch. Notochord, undergoing vacuolisation. 

 r .1 .1 i . f- Eye. ent.c. Enteric cavity, f. Adhesive 



from the others, and it papil ^ ^ Anterior por(io y n O f nerve . tube 



becomes gradually trans- (spinal cord), o. Otocyst, lying on the floor 



of the cerebral vesicle and projecting up 



ferred by a differential freely into its cavity, r.a. Right atrial involu- 

 4.1- r 4.V. 11 r tion. st. Stomodoeum. 



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. 



