340 F. L. LANDACRE 



The process of disappearance is as follows: When the preaudi- 

 tory placode is at its maximum size it extends from the anterior 

 end of the auditory vesicle as far forward as the point where the 

 hyoid gill pocket comes into contact with the ectoderm. Its 

 posterior end is at first continuous with the auditory vesicle and 

 its anterior end gradually thins out into ordinary epidermis. 

 Fig. 14, Stage IV, is taken just anterior to the auditory vesicle 

 at a time when the placode is at its maximum size. The radial 

 arrangement of the cells of the placode and the partial formation 

 of a cavity corresponding to the cavity of the auditory vesicle 

 are evident. This figure should be compared with fig. 6, Stage 

 III. In fact, in some series there is a small cavity in the placode 

 corresponding to that of the auditory vesicle. The first change of 

 a retrogressive nature that I can detect is illustrated in fig. 15, 

 which is taken from the opposite side of the same embryo. Here 

 there is an area corresponding to the thickness of one section 

 intervening between the posterior end of the placode and the an- 

 terior end of the auditory vesicle. Here the placode seems to have 

 broken down into mesectoderm, since we have only mesectoderm 

 with a trace of the ventral portion of the placode left, where on 

 the opposite side the placode and vesicle are continuous. The 

 irregular outlines of the posterior end of the placode and of the 

 anterior end of the vesicle also indicate that the placode has been 

 converted into mesectoderm and that the two structures have not 

 simply been carried apart by the growth of the embryo. It is not 

 likely that the placode has moved forward bodily, since the anter- 

 ior end remains constant in position. 



In the next stage sketched from an embryo of the same age 

 but slightly more developed, there is an area of six sections inter- 

 vening between the posterior end of the placode and the anterior 

 end of the vesicle where the placode has been converted into mes- 

 ectoderm. The placode here (fig. 16, Stage IV) differs somewhat in 

 appearance, being longer in its dorso-ventral axis and resembling 

 less the auditory vesicle. While the nuclei are still situated in 

 the inner extremities of the cells, the placode has the appearance 

 of columnar epithelium. In a somewhat older series (fig. 17, 



