OKIGIN OF THE LATERAL LINE PRIMORDIA 601 



the mesial surface from the dorsal thickening of the first true 

 gill (fig. 28). This separation is not complete as yet but becomes 

 so later, proceeding from in front backward; at the posterior end 

 of the more dorsal thickening the two are confluent. The ventral 

 thickening, it will be noticed from figure 28, occupies the lateral 

 body angle formed by the nearly vertical body wall and hori- 

 zontal body wall. It is an interesting fact that this more ventral 

 thickening becomes partially separated and. is certainly in exist- 

 ence before the endodermic gill pocket reaches the ectoderm. 



This more ventral ectodermic thickening, which for conven- 

 ience can be called the angular thickening, since it lies in the 

 angle at the side of the body, can be best interpreted, at least 

 over the area of contact, as the forerunner of the ectodermic por- 

 tion of the gill slit. This is very slight in an open slit but still 

 is present. It is no more remarkable that ectoderm should un- 

 dergo changes preceding the contact than that the endoderm 

 should produce a well defined evagination before it comes into 

 contact with the ectoderm. The exact position of the more ven- 

 tral portion of this common thickening before it separates from 

 the more dorsal is apparently due solely to the position of the 

 future gill slit. This, as a glance at figures 28 to 34 will show, 

 is quite low on the side of the body and near to the yolk. The 

 term angular thickening as applied to this region of thickened 

 ectoderm may prove to be quite superfluous, If it represents 

 only an early stage in the appearance of each ectoderm gill thick- 

 ening, it is not needed. If, however, it should be concerned in 

 strengthening the side wall of the body at the angle, its use would 

 have more justification. At any rate, it is a convenient means 

 of designating the common thickening out of which the true gill 

 thickenings form and which precedes the contact in appearance. 



Stage XVI. In this stage the auditory vesicle is detached 

 from the ectoderm except at its extreme anterior end. The 

 endodermic pocket of the second true gill touches the ecto- 

 derm over an area corresponding to the anterior end of the 

 more ventral thickening of the preceding stage (fig. 29). The 

 posterior end of the more dorsal thickening extends back over 

 the anterior end of the more ventral (fig. 1), in fact beyond the 



