title: [Morphogenesis and histogenesis of genetically based twin feathers in domestic pigeons (columba livia dom. L.)].
abstract: 1. "Twin feathers" described for various domestic breeds of pigeons have been found in Oriental Rollers in 12 per cent of the sides of the tails during each of the breeding periods. 2. Each studied twin feather originates in a single follicle. It shows either one sheath ("split feather") or two sheaths ("double feather"). 72 per cent of the feathers have been found in the middles of the tails, 23 per cent in the middle of one side of the rows of rectrices. 3. Twin feathers grow either out of one papilla and one blastema having the ability for forming two shafts (split feathers) or from two separated papillae and blastemas lying in a single follicle cavity (double feathers). 4. The degree of splitting of split feathers is correlated to the position of the shaft primordia in the collars. Feathers of a degree of splitting exceeding 85 per cent have been found growing from collars with a pair of shaft primordia in an approximately diametrical and lateral position and a consequently large ramogenous zone between them, otherwise the two rhachis fuse during their development so that the follicle gives rise to a feather of a smaller degree of splitting. 5. The individual formation of a twin feather in a definite follicle will not vary in the succession of the feather generations, except for the rare non-reappearance of one twin of double feathers. Especially the degree of splitting has been observed to be invariable. 6. Generally the material of a twin feather is augmented compared to the symmetrically situated feather of the opposite side of the tail. The weight and area of a double feather is doubled, the weights of split feathers are significantly increased, the areas only of feathers with a degree of splitting exceeding 85 per cent. 7. Similar principles may be stated for twin downs of nestlings, but a great number of twin downs with mutual filaments has been observed. In the first generation of contour feathers, the twin downs are generally followed by twin feathers, only in rare cases by normal feathers. The specific formations of so-called "stunted feathers" (representing a kind of double feathers) may be followed in their turn by double or split feathers, if the malformation will not recur. 8. In cross sections of the basic region of twin downs of nestlings, a fusion of the blastemas and papillae has been found in many cases. It may be supposed that in this way the blastemas of the split feathers, studied in this paper, originate, that means by a down-growth of two primordia into one follicle (caused by an excessive number of primordia in the delimited row of definitive tail feathers) and by a subsequent fusion. In contrast to the follicles of split feathers, the collars and papillae remain separated by a basic portion of the follicle wall in follicles of double feathers.
authors.affiliation: Zoologisches Institut der Universität zu Köln, Deutschland.
authors.forename: Hans-Jürgen
authors.initials: HJ
authors.lastname: Hoffmann