Mitotic chromosomes from the feather pulp and leucocyte cultures of the great horned owl (Bubo v. virginianus) were analyzed in both the sexes. The largest pair of chromosomes are acrocentrics while those of the second and the third pair have a short arm 1/6th the size of the large one. Chromosomes of the fourth and the fifth pairs have a median constriction while those of the 6th to 9th pairs are all acrocentrics. These are followed in order of size by a large number of smaller chromosomes, the smallest of which appear as dots. In 15 plates out of a total of 21, 82 to 84 chromosomes were counted and it is presumed that either of these numbers represents the diploid number. In the female sex besides the single unpaired 4th Z-chromosome, there is a small metacentric unpaired "W" chromosome which is nearly equal in size to the chromosomes of the 7th pair.
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