422 L. S. STONE 



contribution to the formation of the mesoderm and the exact 

 origin and fate of the 'mesectoderm' tissue. Experiments have 

 never before been appUed to the solution of these problems, all 

 of our knowledge having been obtained from studies made upon 

 successive developmental stages of normal embryos. 



In the present study of the early stages of development it has 

 been found that cells are given off for the formation of certain 

 gangha by epibranchial, lateral-line, gasserian, and ophthalmic 

 placodes. From the early appearance of the neural crest in the 

 dorsal portion of the neural canal at the time of the closure of the 

 neural folds the crest cells have been followed as they migrate 

 over the mesoderm of the visceral arches, increase in number and 

 finally arrange themselves upon the median and ventral sides of 

 this mesoderm, where they form the cartilages of the visceral 

 skeleton except the second basibranchial. The distinctness of 

 the early crest-cell groups and placodes makes it possible to re- 

 move these structures by employing the operative methods now 

 so commonly used by investigators upon amphibian embryos. 

 It is also possible to remove areas of ectoderm from early embryos 

 in which the placodes have not yet become prominent enough 

 to be identified, and thus to ascertain how early these placodes 

 in the ectoderm are laid down as definite entities. 



Much confusion arose in the earlier descriptions of placodes 

 by the failure to distinguish clearly between ectodermal thicken- 

 ings in relation to the gill clefts and those related to the forma- 

 tion of the lateral-line system. Not until von Kuppfer ('91) 

 clearly pointed out the distinction between dorsolateral and ven- 

 trolateral, or epibranchial placodes, was there hope of obtaining 

 light on the formation of the components of cranial ganglia. De- 

 scriptions of cells breaking off en masse from the epibranchial 

 placodes and being added to portions of the cerebral ganglia have 

 repeatedly appeared in the anatomical studies of all types of 

 vertebrates, especially among the fishes and amphibians (van 

 Wijhe, '82; Beard, '85; Froriep, '85; Piatt, '96, and Landacre, 

 '10 and '12). Landacre has assigned a definite function to these 

 epibranchial placodes. He has shown in Ameiurus ('10) that the 

 special visceral portion of the IX ganglion appears to come from 



