2 52 
The Ohio Naturalist. 
[Vol. VIII, No. 4, 
placodes or they do contain cells derived from the neural crest or 
the mesectoderm? 
(3) Are the placodes in their early growth and at the time of 
their origin proportionate in size to the communis ganglia of the 
adult? 
(4) Is there any evidence that the placodes arise in response 
to a stimulus furnished by the neural crest portion of the cranial 
ganglia, as has been stated, or is there any evidence that they 
may be in part or wholly endodermic in origin ? 
(5) Lastly, if they are of pure ectodermic origin do they 
give rise to both general and special visceral fibres or only to 
special visceral fibres? 
A brief description of the origin of the placodes will answer 
several of these queries. 
In Ameiurus the placodes appear serially from anterior to 
posterior on the hyoid and first four true gills, none being 
developed on the fifth. 
They arise in the ectoderm just dorsal and posterior to the 
gill slit at the time when the endodermic evagination comes into 
contact with the ectoderm. 
Their presence is first indicated by a thickening of the 
ectoderm; this is followed by a period of active mitosis in the 
ectodermic thickening and a proliferation of cells into the 
mesoderm of the corresponding gill bar: This proliferated .mass 
pushes its way posteriorally and dorsally and finally becomes 
detached from the skin which later assumes its normal thickness. 
The proliferated masses especially in the IX and first two 
divisions of the X nerves are quite definite in outline and stain 
much darker than either the ectoderm from which they arose or 
the mesoderm. This difference in staining seems to be due to the 
amount and character of the intercellular substance. In the VII 
nerve the placode is so large and the proliferation of cells so rapid 
that in its earlier stages at least it is not so definite in outline as 
those of the IX and X, bat in all cases from the time the pro¬ 
liferation begins the cell mass is easy to follow. 
The size of the mass proliferated’ is apparently proportionate 
to that of the communis ganglia of the adult being largest in the 
VII nerve and much smaller in the IX and first two diivsions 
of the X. 
The placode of the fourth true gill combines so closely with 
a neural crest portion that I am unable to compare it with the 
others. 
All these ganglia of placodal origin have been followed up to 
a stage where the roots and chief trunks of the nerves are formed 
and there can be no doubt except in the third division of the X 
that they give rise to the communis ganglia of the adult. 
