604 F. L. LANDACRE AND A. C. CONGER 



The primordium of the lateral line (fig. 4, Po.L.L.) is only 

 slightly developed in this series, but it is quite pronounced in the 

 next series plotted and occupies the same position as in the series 

 under discussion, so that while it is indefinite there can be no 

 doubt as to its identity. This primordium is not the forerunner 

 of the main body line which appears more posteriorly in the next 

 series plotted (Stage XXI), but is the prunordium of the group 

 of lateral line organs supplied by the ramus supratemporalis IX 

 arising from the lateralis IX ganglion. 



The position of the lateral line primordium in the longitudinal 

 axis is coextensive with the posterior fourth of the auditory ves- 

 icle. Dorso-ventrally it lies at the level of the lower border of 

 the auditory vesicle. It is most distinct anterior to the thicken- 

 ing of the first true gill, but seems to extend back over that thick- 

 ening, as indicated in figure 4. It is quite detached from that 

 thickening and lies at a higher level. 



The appearance of this primordium quite distinct from both 

 gill thickenings and auditory vesicle is in harmony with condi- 

 tions in the preauditory region as described in the first section 

 of this paper. The auditory vesicle at its posterior end has 

 been detached from the epidermis for more than thirty-two hours 

 and during all that time there has been no trace of a primordium 

 of the lateral line organs, and there seems to be no doubt that 

 the primordium first arises as a localized differentiation in the 

 ectoderm quite separate from any connection with the auditory 

 vesicle. Its relation to the origin of the lateralis IX ganglion, as 

 well as the relation of the main line to the laterahs X ganglion, 

 is not so clear. No trace of either of these ganglia can be 

 positively identified in this stage. 



Stage XX. Stage XX, which lies in point of time between the 

 last stage and Stage XXI shows no advance on stage XIX. 



Stage XXL Stage XXI (fig. 5) is characterized by the 

 presence of four thickenings associated with the gills and by 

 the presence of two lateral line primordia in addition to the one 

 mentioned in Stage XIX. 



The two additional primordia found in this stage will be desig- 

 nated the second and third postauditory and are associated with 



