POSTURE OF MEMBRANOUS LABYRINTH 175 



developing cartilaginous skeleton, laterally and dorsally by the 

 non-resisting auditory capsule and skin. In front of the prootic 

 ganglion mass and the optic vesicle, and caudally the vago-glosso- 

 pharyngeal complex. These different structures present differ- 

 ent degrees of resisting pressure, and thus from different directions 

 there are these compression forces acting upon the ear vesicle, to 

 which, when normally placed, it is properly adjusted by its shape 

 and the firmness of its different parts. Normally there is an 

 equilibrium between the compression forces of the environment 

 and the resisting forces of the vesicle. When the vesicle is ab- 

 normally placed there is a disturbance in this equilibrium which 

 continues until the vesicle regains its normal position. Thus we 

 would have the mechanical tendency for the disturbed vesicle to 

 fit itself into the right position. An objection to this explanation 

 immediately suggests itself, and that is the fact that vesicles 

 having an abnormal form, and that could not possibly fit well in 

 the usual pocket, notwithstanding, right themselves almost as 

 well as the normally shaped ones. As another mechanical factor 

 one might think of gravity. It is well known that gravity con- 

 stitutes a decisive factor in certain embrylogical processes. Hert- 

 wig ('99) and Wetzel ('04) experimentally produced deviation 

 from the normal development of the egg by alterations in gravity 

 through the use of a centrifugal machine. It is true that gravity 

 controls the position within the egg membranes of the amphian 

 larvae in the early stages. The ventral side, due to the yolk mass, 

 is heavier and is always down. In the case of the ear vesicle the 

 large macula in its floor is thicker and presumably heavier than 

 the other portions of the vesicle wall, and we might assume this 

 fact as the reason that we always find the macula towards the 

 ventral side and thereby a factor in the posture of the vesicle as 

 a whole. However, when the larvae are removed from the mem- 

 branes, as is necessary for purposes of operation, the conditions 

 are quite altered. Being no longer supported by a gelatinous 

 sphere which is easily kept properly erect by gravity, the larvae 

 fall to the bottom of the dish and rest on their side, and we imme- 

 diately have an abnormal direction of gravity, which persists 

 throughout the critical period in the development of the ear ves- 



