456 L. S. STONE 



The groups of sense organs may be described in the following 

 manner: 1) the supra-orhital, a group usually arranged in a double 

 line passing dorsal to the eye then ventral to a region median to 

 the external nares; 2) the infra-orbital, a group sometimes in a 

 single, sometimes in a double row, passing ventral to the eye to a 

 region on the upper jaw ventral to the external nares; 3) the occip- 

 ital, an approximately triangular group of variously arranged 

 organs with a short line of four or five sense organs extending 

 caudally as far as the region above the third external gill; 4) the 

 hyomandihular or postorhital, an irregularly arranged group, 

 usually a double line associated with the infra-orbital passing 

 caudally and ventrally from the latter to the ventral side of the 

 head; 5) the mandibular, a single line of sense organs extending 

 along the side of the lower jaw from the symphysis back to the 

 ventral extremity of the hyomandibular group and giving off, 

 at the angle of the jaw, a branch of two or three sense organs, 

 sometimes called the angular group, extending dorsally to the 

 infra-orbital line; 6) the ventral-hyomandibular group, a double 

 line of sense organs on either side, beginning at the symphysis 

 of the jaw passing median to the mandibular group to meet the 

 latter at its posterior extremity, where it gives off a single line 

 of seven or eight sense organs which passes directly mesially, but 

 does not join with the corresponding branch of the opposite side; 

 7) the midbody line, a single line of sense organs extending from a 

 region above the proximal end of the limb behind the external 

 gills to the region above the level of the anus, where it bends up- 

 ward to pass along the upper border of the somites to the end 

 of the tail; 8) the dorsal body line, a single line of sense organs 

 extending from behind the dorsal end of the third gill along the 

 upper border of the somites, where it joins the midbody line in 

 the region above the anus; 9) the ventral body line, a single line of 

 sense organs curving around the ventral border of the fore limb 

 and extending to the ventral border of the hind limb. 



In this morphological study, in which critical stages have been 

 described, it is obvious that at the time of the closure of the 

 neural folds (stage 21) no definite placodes can be accurately 

 located, unless it be a slight indefinite thickening dorsal to the 



