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 placodt s 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 the}^ 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, but 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 gih 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. 



