364 



F. L. LANDACRE 



In the next series, 75 hours, which I have not figured, little 

 change has taken place, although the placode extends somewhat 

 farther mesially; but in an embryo of 81 hours (fig. 55) a change 

 can be noticed. Figs. 55, 56, 57 are consecutive figures taken from 

 the same embryo. Fig. 55 is taken from the anterior portion of 

 the placode and shows a decided constriction between the gan- 

 glionic mass and the epidermis with which it is still attached. 

 The epidermis immediately dorsal to the attachment of the pla- 

 codal ganglion has resumed its normal appearance, having a rough 



edu/la 



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 ud qanal/on. 



e pi 'branch la t p/a code 

 branch- art 

 'endoderm 



Fig. E. A camera tracing corresponding to fig. 56, showing the position of the 

 ppibranchial placode. NO. notochord; b.v. blood vessel. 



inner border composed of cells which seem to be in process of be- 

 coming mesectoderm. Ventral to the placode the epidermis is 

 still thick and the appearance here, as elsewhere in the formation 

 of epibranchial placodes, indicates that the epidermis ventral to 

 the ganglion is the chief source of whatever cells migrate into it. 



The following section (fig. 56) presents approximately the same 

 appearance except that the nuclei of the epidermis are separated 

 from those of the ganglion which is attached to the epidermis by 

 a cytoplasmic mass only. In fig. 57, however, we find the gan- 

 glionic mass detached from the ectoderm and lying free in the 

 mesectoderm, while the epidermis resembles that of adjoining 



