The feathers in many parts of the body are developed in rows, there being intervals, or elongated skin areas, between these groups of several rows, which are not provided with feather papillæ, but which are covered over by the feathers developed in front of these spaces. The definite feather lines, or areas, have been called _pterylæ_. The intervening tracts devoid of feathers are called the _apteria_. The first outer covering of the bird, or baby chick, is a temporary one consisting of fasciculi of long filaments of down. These fasciculi on their first appearance, are enveloped in a sheath, which soon becomes ruptured and are entirely cast off by the time the baby chicks are ready to be taken from the nest or the incubator. [Illustration: FIG. 80.—Photomicrograph of the skin of the neck of a S. C. White Leghorn hen. 1, The skin possessing an outer stratum, the stratum corneum, next the stratum lucidum, next the stratum granulosum and underneath the stratum germinativum. Beneath the outer dark band representing the upper skin strata is the pars reticularis of the derma. 2, Papilla showing from inside to outside the hyaline feather wall, the stratum corneum, the Malpighian layer of the follicle, the corium. Inside the feather is noted the pulp-like material. ] The down fasciculi, each emerging from its small quill, are succeeded by the feathers, which they apparently guide through the skin. Feathers do not spring from all parts of the body alike; especially devoid of feathers are those parts where chafing and friction is greatest, as under the wings and in the groin. At the end of the quill there is a small opening, the _inferior umbilicus_, into which projects a papilla of the dermis. Where the quill emerges from the skin there is another small opening, the _superior umbilicus_, from which springs frequently a small feather called the _hyporachis_. The shaft, or _rachis_, has a groove extending along that surface which lies next the body.