Plate XXIII. 



Fig. 1. Sec. No. 216. Gang. VII, showing the placodal (PI) thickening of the 

 ectoderm at a point opposite the ganglion. The placodal contribution is small at 

 this point and is characterized by the presence of cells with darkly staining nuclei; 

 there are a number of mitotic figures, indicating active proliferation. Actual 

 contact between the ganglion and the ectoderm is not present in this section. 



Fig. 2. Sec. No. 222. G. VII. The distinguishable portion of the placodal 

 contribution is much larger than in the preceding figure. There is a contact 

 between the ganglion and the ectoderm, and the fact that the most external cells 

 of the contributed mass do not show the characteristic dark nuclei, indicates 

 active contribution of cells by the placode. The most recently contributed cells 

 have not yet undergone the stage of metamorphosis seen in those that have 

 migrated farther into the body of the ganglion. 



Fig. 3. Sec. No. 224. G. VII. In this section, there is a reduction in the 

 comparative size of the placodal component, absence of contact and a reduction in 

 the number of mitotic figures, from the number in previous sections. 



Fig. 4. Sec. No. 227. G. VII. There is still further reduction in the compar- 

 ative size of the placodal component and a comparatively large area of undifferen- 

 tiated cells on the external portion of the ganglion near the ectoderm. 



Fig. 5. Sec. No. 231. G. VII. There is no evidence of actual cellular contact 

 between the placode and the ganglion and, while still very near the placode, there 

 is no evidence of recently contributed, undifferentiated cells on the lateral 

 periphery of the ganglion. 



Fig. 6. Sec. No. 233. G. VII. The dorsal portion of this section and the 

 sections already described, constitute the lateral line component of VII (V. L. 

 VII), the lower portion, the general visceral component; the latter, in this section, 

 occupies an area of about the same extent as the placodal component, though this 

 is reduced from the extent displayed in the previous section. There is still some 

 active proliferation as evidenced by the presence of mitotic figures, not all of 

 which are in focus at this level. 



Fig. 7. Sec. No. 237. G. VII. The body of the entire ganglion is not so clearly 

 defined at this point as in the more anterior and more mature portions. The 

 placodal thickening is not greatly reduced and there is evidence of very active cell 

 proliferation, though actual continuity of cellular elements is doubtful. 



Fig. 8. Sec. No. 216. G. VII. An outline drawing of the same section as Fig. 1, 

 showing the general relations of the area. The lateralis component is relatively 

 smaller than the general visceral. The relation of the entire ganglion and the 

 placode to the first gill cleft is clearly seen. There can be no doubt as to the 

 independence of the placodal thickening from that situated at the opening of the 

 gill cleft. 



Fig. 9. Sec. No. 224. G. VII. An outline drawing of the same section as Fig. 3. 

 The branchial cleft has deepened and the gill cleft thickening of the ectoderm is 

 consequently carried farther away from the placodal thickening, which still 

 maintains the form and size displayed in the intervening sections, as well as in 

 those situated more posteriorly. 



