2? F, L. LANDACRE 
cells which also disappear later. The placodal portion of the 
ganglion always rests upon the dorsal endodermic prolongation 
and in earlier stages is in continuity with the ectoderm at the pos- 
terior end of the dorsal endoderm process and abuts against it 
as shown in model (fig. 33). In the 10 mm. embryo where the 
placode is no longer in contact with the ectoderm and is separ- 
ated from it by the primordium of the hyomandibular cartilage 
it still rests upon this dorsal pocket. The later history of the 
placodal cells is closely associated with this pocket. 
THE LATER HISTORY OF THE PLACODE OF THE VII 
The later history of the placode may be summarized briefly 
as consisting of (a) the withdrawal from the epidermis and incor- 
poration into the geniculate ganglion, and (b) the reduction in 
number and the metamorphosis of the numerous compact pla- 
codal cells into ordinary ganglion cells that cannot be distinguished 
from cells of the geniculate ganglion derived from the neural 
crest. As to the withdrawal from the epidermis, a comparison 
of figs. 22, 23, 24, 25, with figs. 17, 18, 19 will show that the pla- 
codal spur of the geniculate ganglion nowhere approaches so 
closely to the epidermis as in the earlier stage. Fig. 25 shows 
also that the posterior end of the lateral spur extends caudad 
from the ganglion and lies, as an elongated mass of cells, small 
and dark staining like the placode, upon the endoderm of the 
hyoid pocket. The appearance of the posterior extension of 
cells is usually like that in fig. 25. The appearance of the caudal 
prolongation changes slowly. It is gradually withdrawn into 
the ganglionic mass of the geniculate. Fig. 30 illustrates the con- 
dition at the middle of the placodal mass and fig. 29 illustrates 
the condition five sections caudad where there are no placodal 
cells in the ganglion but the posterior spur is present (12.4 mm.). 
In stage 13.5 mm. there is no longer any posterior projection of 
placodal cells although the placodal cells reach the ventral sur- 
face of the ganglion (fig. 31). 
The later history of the placodal cells is rather peculiar. They 
are the most striking feature of the facialis complex. The cells 
