208 MARINE BIOLOGICAL LABORATORY. 



quadrangular shape. This condition is found in embryos 

 of tliree and a half to four weeks. The utricular and 

 saccular divisions are readily distinguished, and even 

 now the semicircular canals have begun to differentiate 

 by the formation of two shallow depressions in the walls 

 of the utriculus. Both the pocket common to the 

 vertical canals and the pocket for the external canal 

 are present. Below the latter is seen the larger and 

 deeper evagination for the cochlea. When the embryo 

 has reached the length of 8 mm., the cochlea has so far 

 developed as to stand out distinctly from the sacculus, 

 and is at the same time bent (Fig. it, /). At both 

 ends of the common pocket of the verticals, and at 

 one end (anterior) of the external depressions, are slight 

 enlargements — the future ampullae. From now on, the 

 parts rapidly acquire individuality. The anterior canal 

 is completed first. Then the posterior becomes cut off, 

 and soon after the external is perfected. 



The whole vesicle undergoes a marked change in form 

 during this period, for, owmg to the growth of the 

 ductus endolymphaticus from the apex of the primitive 

 vesicle, and the great elongation of the cochlear tube 

 from the bottom of the saccular region, the canal com- 

 plex now appears drawn out in a dorso-ventral direction. 

 The cochlear canal has now, about the fifth week, one 

 half a spiral turn, and a long groove has appeared on 

 the inside of its wall (fold on the outside), from which 

 the nerve end-organ — Corti's organ — arises. In the 

 embryo of 30 mm. (Fig. 6) length the canals are well 

 formed, and the ampullae are quite prominent. The 

 two verticals which from tJieir viode of development 

 have up to this time occupied the same planCy now begin 



