354 • B. F. KINGSBURY 



The epibranchial placodes 



While the growth shiftings in the cephalo-caudal growth in 

 head and neck produce the effects upon the branchial pharynx 

 just described, these are modified and correspondingly influenced 

 by the growth and expansion in the transverse dimensions. In 

 this connection it will be necessary to call attention to the effects 

 produced by the expansion of the branchial arches II, III and 

 IV, particularly the second arch, which through its growth, 

 together with that of the postbranchial and epibranchial regions, 

 outhnes a roughly triangular depression and determined its 

 transformation into the sinus cervicalis, into which the second, 

 third and fourth clefts open. It would be entirely superfluous 

 to describe, in repetition of others, the further transformation 

 of the sinus cervicalis into the ductus branchialis II and the 

 vesicula cervicalis, save in so far as it is necessary to emphasize 

 the fact that they are formed in the increase in thickness as a 

 result of a growth whose relations, direction and degree they 

 express, and to call attention to the influence which the epi- 

 branchial placodes have in their formation. 



These last named structures have until recently received small 

 recognition and httle emphasis in texts of embryology when it 

 is considered how lo'ng their occurrence has been described. Since 

 von Wijhe ('82) described them in the elasmobranch, and Froriep 

 ('85), under the name of 'Schlundspaltenorgane,' in the mammal, 

 they have been described in elasmobranchs (Froriep '91), the 

 lamprey (v. Kupffer '91), teleosts and ganoids (Landacre '12), 

 amphibia (Landacre and McLellan '12; Driiner '04^), reptiles 

 (Lucy W. Smith*), birds* (Kastschenko '87, '10), and in man 

 (Streeter '11, Hammar '11). In all instances they bear a rela- 

 tion as epibranchial thickenings to clefts I to IV (or more), while 

 the ventral (caudal) portions of the respective ganglia i,VII, 

 IX, X) are intimately fused with them. In the lower forms 



^ Driiner clearly describes the epibranchial placodes in the axolotl and recog- 

 nized that they contributed to the ganglia of the Vllth, IXth and Xth, although 

 he described them as 'ectthymus' (p. 552). 



^ From unpublished observations made in the laboratory by Miss Lucy W. 

 Smith, in the turtle, Chyrsemys marginata, and Miss M. E. Goudge, in the chick. 



