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



3. The structures lying on a level with the auditor}^ vesicle 

 and posterior to it are: (a) A common thickening of the ecto- 

 derm at the side of the body where the more or less vertical body 

 wall becomes horizontal when it turns laterally to spread out 

 over the yolk. This is a common primordium out of which the 

 various gill thickenings develop and may have no significance 

 aside from the development of these thickenings, (b) The ec- 

 todermic thickening at the point of contact of each endodermic 

 gill pocket with the ectoderm, (c) The posterior extensions of 

 these thickenings behind the area of contact of each gill, (d) 

 The epibranchial placodes developed as mesial extensions of each 

 thickening at the dorsal and posterior border of each gill slit, 

 (e) The primordia of the lateral line, of which there are three. 

 In the first of these develop lateral line organs supplied by the 

 ramus supratemporalis IX; in the second organs supplied by 

 the ramus supratemporahs X; in the third organs supplied by 

 the main lateral line nerve of X. 



4. The angular thickening, as it has been designated in the 

 body of the paper, can be best described in connection with the 

 gill thickenings. This is the first thickening of the ectoderm to 

 appear in the postauditory region and is a longitudinal thicken- 

 ing of the ectoderm over the area in which endodermic pocket 

 of the first true gill will come into contact with the ectoderm. 

 This thickening appears, at least in the case of the last four gills 

 and probably in the case of the first, before a contact is formed. 

 After a contact is formed between the endoderm and the ecto- 

 derm in the case of the first gill the anterior end of the first thick- 

 ening is spht into two portions, the more dorsal of which repre- 

 sents the area of contact of the first true gill, arid the more 

 ventral of which represents the area of contact of the second 

 true gill and lies at the extreme lower border of the lateral wall of 

 the body. Posterior to this area of contact of the first true gill, 

 both thickenings are continuous and much broader dorso-ven- 

 trally than either of the separated portions at the anterior end. 



The endodermic pocket of the second true gill comes into con- 

 tact with the more ventral of the two thickenings mentioned 

 above. At the posterior end of this second thickening it becomes 



