ORIENTATION OF THE EAR VESICLE 19 



of the first, being placed so that its lateral surface would lie 

 against the brain. The operation was performed at the end 

 of the non-motile stage. It may be mentioned here that a slight 

 inequality in the development of the larvae at the time of operation 

 is almost unavoidable. The movement of the larvae at the time 

 of operation, which is a source of annoyance to the operator, 

 may be easily checked by placing a rusted needle in front of them. 

 Only the right side of the animal was used for operation. For 

 a study of the first problem, the specimens (Rana palustris) 

 were fixed at various periods after operation, from fifteen minutes 

 to thirty-four hours. For the second, I transplanted the ear 

 v^esicle of Amblystoma to Rana, and vice versa, in reversed 

 position in the emptied ear pocket. Fixation was done two weeks 

 after operation. In the third, the ear vesicles were taken out 

 and put back in the same pocket upside down, without changing 

 the posture in regard to the vertical axis. In this case trans- 

 plantation was not performed, because there exists no character- 

 istic to denote the upper or lower side of the vesicle, and therefore 

 mistakes might easily occur when a vesicle is carried from one 

 specimen to another. 



Mueller's fluid was used exclusively for fixing purposes. The 

 specimens were imbedded in paraffin and cut 10/x thick. In the 

 first study all sections containing the operated ear vesicles were 

 drawn serially at a magnification of 100 diameters. Recourse 

 to these drawings greatly facilitated the study of the experiments. 



RESULTS OF EXPERIMENTS 



All of the larvae used were in an early stage of development 

 when the ear vesicles present no sign of definite differentiation — a 

 condition which makes the determination of their position diffi- 

 cult. The criteria necessary to decide this question constitute an 

 important consideration. 



In very early stages of normal development the ear shows as 

 an invaginated cup derived from the deeper layer of the skin. 

 In the next stage the cup is pinched off, its edges close in, and 

 finally a complete vesicle is formed. The medial and ventral 

 parts of its wall consist of high columnar cells, the nuclei of which 



