596 F. L. LANDACEE AND A. C. CONGER 



The 154-hour stage (fig. 6), shows a marked development in 

 the sensory ridges. A description of the sensory Unes in this 

 stage has ah-eady been given under the the general description 

 of the 10-mm. embryo (p. 583), to which the reader is referred. 



/. The primordium of the hyo-mandihular line 



Owing to the acute angle at which the transverse sections cut 

 the hyo-mandibular line, the radial arrangement of the cells is 

 not so pronounced as in the supra-orbital and infra-orbital lines, 

 and for that reason the hyo-mandibular line is less easily 

 recognized than the other lines anterior to the auditory vesicle. 



In the 106-hour stage (fig. 4) a mass of cells, scarcely differ- 

 entiated from the contiguous ectoderm, may be observed at the 

 level of the roof of the pharynx, and directly beneath the line of 

 contact of the ectoderm and endoderm in the region of the hyoid 

 gill pocket. The structure can be identified in three sections, 

 since its cells project farther mesially than do the ectodermal 

 cells anterior and posterior to this region. An examination of 

 later stages show this to be the first appearance of the hyo-man- 

 dibular line. In the 120§-hour stage (figs. 5 and 20), the hyo- 

 mandibular line has grown cephalad and sharply ventrad, so that 

 it may be traced from the region immediately below the hyoid 

 contact and at the level of the roof of the pharynx until it is lost 

 in the thickened ectoderm in the lateral angle where the embryo 

 joins the yolk sac. 



The position in which the primordium of the mandibular line 

 arises is so far ventral to the primordium of the supra- and infra- 

 orbital lines that it furnishes strong evidence of its independent 

 origin; furthermore, it is separated from the dorsal primordium 

 of the lateral line by the thickening of the epidermis including 

 the hyoid thickening, the epibranchial placode and the posterior 

 extension of the epibranchial placode (fig. 4). Even if inter- 

 mediate series should show it to be farther dorsal' than indicated 

 in figure 4, the presence of these structures is a barrier to conceiv- 

 ing it as having a common origin with the dorsal primordia or 

 the preauditory placode. 



