GENESIS OF FAULT-BARS IN FEATHERS. 335 



the constrictions in unexpanded germs the epidermal layers in- 

 vade the parts normally occupied by the dermal pulp has also 

 been referred to (PI. XII., Fig. 3). Only another word concern- 

 ing the histology of the defects need be said at this point, and the 

 subject will be more fully discussed along with the cause of the 

 defects. The normal development and histology is too consider- 

 able a subject to be considered here and the reader is referred to 

 the papers of Studer, 1 Waldeyer, 2 Davies, 3 Haecker, 4 Maurer, 5 

 Strong 6 and others 7 for this information. 



Much of feather structure is indicated in the plates of the 

 present paper, but the description will deal only with such 

 structures as are directly concerned with our own problem. 

 The general relations of the parts of a feather in cross-section are 

 shown in PI. XII., Figs. 2 and 5. 



The fault-bars in their earliest stages are indicated by a loose 

 union of scattered cells in that part of the intermediate cell-layer 

 which is forming the barbule cells. The cylinder-cell layer and 

 apparently the more central cells of the intermediate cell layer 

 (i. e., those last formed from the cylinder layer) are crowded 

 together as usual. PI. XV., Fig. 26, represents some cellular 

 relations at the close of a fault-bar producing period. 



The Extent and Distribution of the Fault-Bars. 



It has been stated that Professor Whitman was the first to 



direct attention to these abnormalities. He had observed them 



in some of his pigeons ; one of these birds, a hybrid, furnished 



the material for Strong's 8 description of two types of defects 



1 Studer, T., "Die Entwicklung der Feder," Inaug. -Dissert., Berne, 1873. 

 " Beitrage zur Entwicklungsgeschichte der Feder," Zeit. f. wiss. Zool., Bd. 30, 

 1878. 



2 Waldeyer, W., " Untersuchungen ueber die Histogenese der Horngebilde, ins- 

 besondere der Haare und Federn," Beitrage 2. Anat. u. Embry. ah Festgabe T. 

 Henle, Bonn, 1882. 



3 Davies, H. R., "Die Entwicklung der Feder und ihre Beziehungen zu andern 

 Integumentgebilden," Morph. Jahrb., Bd. 15, 1889. 



4 Haecker, V., "Ueber die Farben der Vogelfedern," Arch. f. Micr. Anat., Bd. 

 35, Heft 1, 1890. 



5 Maurer, F., "Die Epidermis und ihre Abkommlige," Leipzig, 1895. 



6 Strong, R. M., "The Development of Color in the Definitive Feather," Bull. 

 Mus. Comp. Zool. at Harvard, Vol. XI., 1902. 



7 A very satisfactory bibliography to 1902 is given by Strong. 



8 Loc. cit. 



