150 GEORGE L. STREETER 



which there is, in addition to the passive pressures of adjacent 

 structures, a motive element, the force of placement, which helps 

 to maintain, and corrects if necessary, the relative position of an 

 individual organ. This force of placement may be defined as the 

 product of the combined motive forces interacting between an 

 organ and its environment. We are already familiar with the 

 striking phenomenon of muscle migration and the movement of 

 nerve-ganglion masses. We can now add to this the potential 

 movement and adjustment of entire organs during their course of 

 development, and it is not necessary to add that this autostatic 

 tendeiicy is doubtless much more pronounced in some organs than 

 in others. 



The writer's attention was first directed to the possibility of 

 experimentally altering the position of the developing labjTinth, 

 during some investigations on equilibration in amphibian larvae 

 (Streeter '06). In these experiments it was found that the cells 

 constituting the ear vesicle are specialised very early, and though 

 transplanted to an abnormal environment they continue to 

 differentiate themselves in the usual way into a recognizable 

 labyrinth. Lewis ('07) showed in a subsequent paper that this 

 was true while they were still in the stage of an uninvaginated 

 plate, quite in contrast to the surrounding cells forming the car- 

 tilaginous capsule, which Lewis clearly proved were not prede- 

 termined in this way. 



In a later paper (Streeter '07) additional evidence was given 

 of the high degree of developmental independence possessed by 

 the early labyrinth cells. It was shown that even fragments of 

 the vesicle may develop independently of the rest of the vesicle, 

 and any individual part, for example the endoljaiiphatic append- 

 age, may be quite normal in cases where the remainder of the 

 labyrinth is very abnormal. It was also shown (Streeter '09) 

 that when two primitive vesicles are crowded together into the 

 same pocket they do not fuse and form together one large laby- 

 rinth, but remain as two distinct labyrinths. Moreover, it was 

 shown that this developmental independence of the vesicles 

 extends to the difference existing between a right and left-sided 

 organ. The dextral or sinistral character, or laterality of the ear 



