AND HAIR-CELLS IN THE DEVELOPING ORGAN OF CORTI. 127 



outer spiral rows. The number of cells of the two boundary rows is more difficult 

 to determine, although in surface views of the membrana reticularis their free apices 

 can be readily enumerated. In this respect the number of inner supporting cells 

 of the second row (figs. 10, 12 and 13, is") corresponds to that of the neighboring 

 acoustic elements (ih), while in the cochlea of the bat, according to N. Van der 

 Stricht, and also as illustrated in figure 10, the original number of apices of the cells 

 of Deiters of the third row (d m ) is just double that of the neighboring hair-cells 

 (oh'"), while in figure 5 (d ui ) it is the same as the latter, and in figure 13 there are 

 four sensory apices (oh ui ) in contact with five sustentacular fields (d iu ). On the 

 other hand, in the adult bat there are just as many sensory apices as sustentacular 

 fields; hence it is plain that in the course of development the number of cells of the 

 outer spiral boundary row is reduced. Further investigation is required to clear up 

 the significance of this reduction. It seems possible, even probable, that this row 

 represents a fourth mixed spiral row, which in man and in some animals may become 

 differentiated into hair and supporting cells. 



(3) The apices of the inner sustentacular elements of the first row resemble 

 those of the cells of Deiters of the first and second rows; they are also even more 

 compressed between the apices of the inner acoustic elements, so that from being 

 originally phalanx-shaped (figs. 12 and 18, is 1 ) they become much thinner (fig. 13 

 is*), and later are veiled by a deeply staining covering derived, according to previous 

 investigations (0. Van der Stricht, 1918), from the surrounding terminal bars. 



The apices of the sustentacular elements of the second inner row resemble those 

 of the cells of Deiters of the third outer row. Indeed, in the first stage of develop- 

 ment both are represented by polygonal fields (fig. 3, is", d 1 ") of about the same 

 size, but the former (is") may be a little larger, particularly more elongated in a 

 spiral direction (figs. 10 and 12, is") owing to higher pressure from the inner hair 

 cells (ih). In more advanced stages (fig. 13, is") they are represented by a very 

 narrow, lanceolate field containing a central corpuscle and circumscribing the inner 

 border of the acoustic elements. In the adult cochlea of the bat (Vespertiliofuscus, 

 Pipistrellus subflavus) , of the dog, and of the rat, this streak or line is covered also 

 by a deeply staining veil produced by the terminal bars (the extension of the terminal 

 bars over these fields is noticeable in figure 13, is"), and the spiral row of apices of 

 cells is then seen in the form of a dark homogeneous streak deeply stained by iron 

 hematoxylin, which constitutes a very sharp demarcation between the membrana 

 reticularis and the large polygonal fields (fig. 13, nd) belonging to undifferentiated 

 columnar cells. 



The apex of the third row of cells of Deiters maintains more or less its original, 

 polygonal shape (fig. 13, d iu ) ; it covers a very dark triangular, fibrillated band or 

 plate, the summit of the apical filament of the sustentacular element, as described 

 by Held (1902) and N. Van de Stricht (1908). 



Baginsky (1886) recognizes below the deep extremity of the inner hair-cells two 

 other elements, one situated internal and the other external to the inner acoustic cell; 

 both are connected with the surface, the former by a process running along the inner 



