ORIGIN OF THE LATERAL LINE PRIMORDIA 609 



against the posterior region of the ectodermic gill thickening. 

 The two ectodermic thickenings are, however, quite different his- 

 tologically in that the preauditory placode resembles the audi- 

 tory vesicle. The preauditory placode next degenerates from 

 before backwards and finally disappears about the time the first 

 trace of the lateral line primordia can be detected. The last 

 trace of the preauditory placode was observed at the anterior end 

 of the auditory vesicle and the first trace of the lateral line pri- 

 mordia appears just over the middle of the hyoid gill thickening 

 having a considerable area of unmodified epithelium between 

 them. When followed in a close series there is no indication 

 of a connection between the preauditory placode and the 

 primordium of the sensory lines. 



4. The hyoid ectodermal thickening appears simultaneously 

 with or previous to the contact of the hyoid gill with the ecto- 

 derm. During the time of contact of the gill pocket with. the 

 ectoderm the thickening extends the whole length of the contact 

 and also projects anterior to the area of contact. This anterior 

 extension however varies little in extent during the whole exist- 

 ence of the thickening and seems to have no special significance; 

 at least, it is not a source of confusion. 



Of more importance is the existence of the posterior extension 

 behind the area of contact. This is present in the very early 

 stages of the thickening and at its posterior end abuts against 

 the anterior end of the preauditory placode, so that if it were not 

 for the difference in histological structure one could quite easily 

 read the preauditory placode forward through the whole extent 

 of the contact of the hyoid gill with the ectoderm. That this is 

 not the case is shown by the histological structure as mentioned 

 above, as well as by the different fates of the two structures. 

 The behavior of similar thickenings in the case of the five true 

 gills renders this conclusion practically certain. 



The posterior extension of the hyoid gill thickening persists 

 throughout the whole existence of that thickening, which lasts 

 up until well defined lateral line primordia are present in both 

 preauditory and postauditory regions. It disappears finally with 

 the last remnant of the gill thickening. 



