426 



JAMES ROLLIN SLONAKER 



{N). In the thickened portion and in contact with the oil 

 droplet is the ellipsoid body {El) described by Greeff ('97) and 

 many other investigators in other species of birds. This oc- 

 cupies ahnost half of the length of the inner segment. 



The external segments of the cones (E) are rather broad at 

 the base and taper gradually to a cone-shaped point. The 

 ratio of their length to that of the inner segments is approxi- 

 mately 2 to 3. 



Fig. 36 Enlarged drawing of the rod and cone elements and their nuclei- 

 C, cones; E, external segments of the rods and cones; El, ellipsoid body of the 

 cone; I, internal segments of the rods and cones; L, limiting membrane; N, 

 nuclei of the rods and cones; 0, oil droplet at the junction of the inner and outer 

 segments of the cones; R, rod; T, twin cones. X 975. 



Twin cones, first observed by Hannover ('44) and described 

 in many species of birds and in other vertebrates by num^erous 

 writers, are common in the sparrow. They consist of two cones 

 which appear to have been crowded together so as to conform to 

 each other's contour. No space exists between their inner 

 segments, but their external segments usually are separated 

 (fig. 36, T). A line of separation between them can usually be 

 seen. Each has a distinct nucleus, which may lie adjacent to or 

 separated from the other. The anatomical structures indicate 

 conclusively that each is an independent element and that they 

 get the name of twin or double cones because of the close 

 relationship. 



