DEVELOPMENT OF THE ORGAN OF CORTI 307 



boring sensory elements, these processes are more numerous 

 (previous, paper) and larger than those of the first and second 

 supporting rows. Situated outside the hair cells of the third 

 row, they are not squeezed and impeded in their lateral ex- 

 pansion like the others. It is not to be wondered at that the 

 products of secretion or cytoplasmic disintegration around the 

 apical fibrillated bundles are more abundant and result in an 

 expansion of the fourth interstice. Nevertheless, the process of 

 development is identical with that of the two preceding spaces 

 and therefore the application of a special term, external tunnel, 

 to designate this formation is unnecessary. However, there can 

 be no doubt that phalanx processes of the cells of Deiters of the 

 third row retain their cytoplasmic sheath much longer than the 

 others, and may show parts of it in the adult cochlea, as pointed 

 out by Held ('02) for the 'apical type' of these cells in guinea- 

 pig, cat, dog, and even the mouse. In such cases these processes 

 are in closer connection with the outer wall of the space than 

 with the medial. 



In the basal spiral turn of the cochlea of a kitten twelve days 

 after birth (fig. 12), the floor of the second, third, and fourth 

 spaces of Nuel is fonned by parts of segments of the cel^s of 

 Deiters {d\ d^\ d''') supporting their corresponding sensory 

 elements {oh\ o}v\ oh'''). The medial and lateral boundaries 

 of the second interstice are represented, respectively, by the 

 lateral surfaces of the hair cells of the first row and the medial 

 surfaces of those of the second. The medial and lateral boundaries 

 of the third interstice are represented, respectively, by the lateral 

 surfaces of the hair cells of the second row and the medial sur- 

 faces of those of the third. The adjoining surfaces of the 

 acoustic elements of each sensory row are separated by narrow 

 clefts, through which all of the spaces of Nuel intercommunicate. 

 These channels, originally occupied by the phalanx processes of 

 the sustentacular cells (those of the first sensory row being the 

 superficial segments of the outer pillars), are liberated after the 

 shifting of the phalanx processes. At first very narrow and 

 virtually obliterated by the process of enlargement of the hair 

 cells, these intercellular clefts become wider by the reduction in 



