ORIGIN OF THE LATERAL LINE PRIMORDIA 591 



106-hour stage (fig. 4) shows an actual decrease in the extent of 

 the thickening, and the contact of the endoderm and ectoderm is 

 only 80 n in length at this stage 



The period between the 106-hour stage and the 120|-hour 

 stage is characterized by a marked decrease in the extent of the 

 ectodermal thickening. The structure becomes rhomboidal in 

 outline and is elongated in the antero-posterior axis, which makes 

 an angle of twenty-eight degrees with the roof of the pharynx. 

 The character of the thickening at this stage is shown in figure 

 5. The actual contact of the ectoderm and endoderm is 50 n 

 in length. The sensory lines are well-developed in the 120^- 

 hour stage and lie somewhat dorsal to the ectodermal thickening. 



c. Epibranchial placode 



The third structure which lies in the region between the audi- 

 tory vesicle and optic vesicle is the epibranchial placode. It can 

 be recognized easily by the small size and dark-staining qualities 

 of its cells. The epibranchial placode can first be identified 

 about the time the preauditory placode disappears and during the 

 maximum development of the ectodermal thickening or contact in 

 the region where the endoderm of the hyoid gill pocket approaches 

 the epidermis. 



In the 94-hour stage (fig. 2), the ectodermal thickening at the 

 posterior end of the contact of the hyoid gill pocket and ecto- 

 derm extends farther mesially than in sections anterior to that 

 point, and an examination of later stages shows this to be the 

 first appearance of the epibranchial placode. In the 106-hour 

 stage (fig. 4) the epibranchial placode can be positively identified, 

 and from this stage the further development of the structure 

 can be traced until it finally is detached from the ectoderm and" 

 joins the general visceral portion of the VII ganglion. In the 

 106-hour stage, the epibranchial placode consists of a mass of 

 cells projecting mesially in the region of the ectoderm formerly 

 occupied by the posterior extension of the ectodermal thickening. 



The 112-hour stage exhibits little change in the epibranchial 

 placode, and aside from a gradual increase in size and the darker 

 blue stain taken by the cells, the placode shows little change 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 23, NO. 6 



