292 G. E. COGHILL 



the skin docs not seem to be understood. Landacre ('12) ob- 

 served the same rehitions in Lepidosteus, but, apparently, did 

 not satisfy himself concerning the meaning of it. 



The ectodermal thickenings which Landacre and Conger have 

 differentiated from the epibranchial placodes and the lateral 

 Hne primordia certainly will be found also in Amblystoma, al- 

 though my studies have not extended to the details of their 

 relations. This subject deserves exhaustive study from mate- 

 rial selected for the purpose. My object in touching upon this 

 topic has been merely to bring the salient features in the differ- 

 entiation of the nervous system which these structures represent 

 into correlation with physiological processes in the organism as 

 a whole. Limited as this study has been, however, I have been 

 increasingly impressed with the results of Landacre's studies on 

 this subject as fundamental to a broadly biological view of the 

 development of the nervous system of the vertebrate head. 

 These ectodermal thickenings and proliferations impress me not 

 simply as vestiges of an ancestral metamerism or branchiomerism, 

 but as centers of physiological processes which may be essential 

 factors in the regulation of the growth of the nervous system. 

 There is a field here for experimental studies, it seems to me, 

 which might prove as illuminating as the well known observations 

 upon the correlative development of the lens and the retina, 

 and it is to be hoped that the purely embryological studies of 

 these structures may be extended to other types of vertebrates. 



2. The junction of the auditory vesicle 



Streeter ('06) has studied the development of the ear as corre- 

 lated with the development of the function of equilibration in 

 the tadpole of the frog. He found that shortly before tadpoles 

 acquire the ability to maintain an upright position ''the labyrinth 

 consists of a closed epithelial sac incompletely subdivided into 

 compartments and possessing differentiated nerve endings which 

 are connected with the brain by the acoustic nerve and ganghon." 

 The semicircular canals are "in the process of development, but 

 are not completely pocketed off until after equilibration is already 



