what are the pattern of developmemt in mammals
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Abstract
The principal points of this paper may be summed up as follows: In mammals and birds that normally are completely pigmented, there are certain definite points of the body from which as centers the tendency to develop pigment in the epidermal structures may become less and less. Outward from each of these centers pigment formation spreads to include very definite areas which in wholly pigmented animals overlap slightly at their borders or are at least contiguous. 2. A reduction in the area covered by any of these primary patches results in a white mark at the line of junction of two contiguous color patches, where no pigment is produced. These white marks between the primary patches are spoken of as primary breaks. 3. Through a study of the breaks in pied individuals of domesticated species of mammals and birds, the boundaries of the primary patches have been determined. These are homologous in the two groups and subject to a certain amount of variation in different types. They are: a median crown patch unpaired, and five paired patches on the opposite sides of the body, which are named from the general areas they cover, the ear, neck, shoulder, side and rump patches. Their limits are more precisely defined under the different species treated. 4. These patches are physiologically independent of each other and may be differently colored in the same individual. 5. Pied patterns among many wild species have been brought about through the areal reduction of these pigment patches in a definite way so that the white markings resulting as breaks between the reduced patches have become fixed and form a permanent part of the normal pattern. 6. In several wild species this development of white markings is shown to be even now taking place, but the amount of pigment reduction is still fluctuating so that the white markings vary much in extent with different individuals. 7. The development of such white markings takes place probably by little and little, so that the departure from type is not so great as to arouse antagonism against the varying individual on the part of others of its species. Also, the gradualness of the change allows the species to become accommodated to any disadvantage that might concomitantly arise. 8. The converse of this centripetal style of pigmentation is present in many species, and results in pigmentation (commonly black) at the extremities or along lines where primary breaks occur in the centripetal form, namely at the tip of the nose, ears, tip of the tail or the toes; possibly the black dorsal stripe is due also to centrifugal pigmentation. Patterns may develop as in certain antelopes by a white break between patches of the two types
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hellow
In developmental biology, pattern formation describes the mechanism by which initially equivalent cells in a developing tissue in an embryo assume complex forms and functions. ... In this context, a field of cells is the group of cells whose fates are affected by responding to the same set positional information cues.
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