THE EPIBRANCHIAL GANGLIA OF LEPIDOSTEUS 23 
as mentioned above stain a deep blue and in addition are quite 
small and closely packed. In a24 mm. embryo they can be iden- 
tified still, although reduced in numbers, and they seem to be 
represented in a 44 mm. and even in a 6-inch Lepidosteus. That 
they should remain so long undifferentiated is striking, but there 
ean be no doubt as to the continuity of this mass of cells and that 
they are derived from the placode. That they are gradually 
transformed into ganglion cells seems evident from the conditions 
in the older series where they are of various sizes and some of 
them are evidently ganglion cells. 
In a 24 mm. embryo the number of undifferentiated placodal 
cells is much reduced and there are groups of similar small cells 
in the posterior portion of the Gasserian, so that the minute cells 
are not peculiar to the placodal ganglion. The process of differ- 
entiation seems to take place on the periphery of the placodal 
cells where they are in contact with the normal ganglion cells. 
Since some placodal cells remain undifferentiated up to the 6-inch 
stage, it is possible that they may never be converted into normal 
cells, although this is improbable. The transformation of these 
placodal cells into ganglion cells is roughly correlated with the 
relative time of appearance of the placodal ganglia. They are 
the last ganglia to be formed and it is not surprising that they 
should transform into normal cells much later, but it is a little 
surprising that they should be delayed so long in their transfor- 
mation. 
THE ORIGIN OF THE EPIBRANCHIAL PLACODE OF THE VII 
The determination of the exact time of appearance of the 
epibranchial placode of the VII nerve is rendered difficult by 
the presence of three associated structures, viz; the preauditory 
placode, the thickening of the epidermis at the point where the 
hyoid endodermie gill pocket joins the epidermis, and lastly the 
presence of the anterior end of the geniculate ganglion which 
ends at the point where the epibranchial placode is formed, 
rendering difficult the identification of detached groups of cells 
lying in this immediate region. 
