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



region of contact of the second gill pocket, and the two merge 

 into a rather long thickening dorso-ventrally but separated by a 

 marked groove. • 



The two thickenings as shown in figure 1 do not represent 

 exactly the same relation in respect to the contact with their 

 respective gill pockets. The dorsal thickening represents chiefly 

 the extension of the thickening posterior to the detachment of 

 ectoderm from the endoderm pocket, while the more ventral in- 

 cludes at its anterior end also the area of contact between the 

 ectoderm and the endoderm. The anterior portion of this thick- 

 ening will later be included in the gill slit and only the posterior 

 will remain as thickened cord in the ectoderm. Since the object 

 is to trace all thickenings that could posibly be taken for lateral 

 sensory lines, this figure is given just as it appears on the slides. 



Stage XVII. In stage XVII the auditory vesicle is completely 

 detached from the ectoderm. The more dorsal thickening of the 

 preceding stages (the first gill thickening) is entirely separated 

 even at the posterior end from the more ventral (figs. 2 and 14 

 for detail) which is associated with the second true gill. The 

 greater portion of the dorsal thickening, a length of nine sections 

 out of a total of thirteen, lies behind the area of contact of the 

 first true gill and its cells take a dark stain, a peculiarity which 

 precedes the formation of the epibranchial placode previous to 

 the detachment of the epibranchial or special visceral ganglion. 

 However, not all of this as it is found in this series is concerned 

 in the formation of the epibranchial ganglion, only the anterior 

 end being so used, the posterior end disappearing. 



The more ventral thickening (fig. 2) is longer than in the pre- 

 ceding stage (fig. 1). It is broader at its anterior end than at 

 the posterior end and there are indications of a division into two 

 parts (fig. 30), although this is not so pronounced as in the next 

 stage. So far as a third ventral thickening can be distinguished, 

 it is the thickening at the angle of the body as in the early stage 

 of the second thickening for the second true gill. As in the pre- 

 ceding reconstruction (fig. 1) the more ventral thickening of fig- 

 ure 2 consists principally of contact area where the second true 

 gill touches the ectoderm. Behind the area of contact of the 



