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Updated: May 31, 2025
With respect to the origin of the hair or wool fibre, this is formed inside the follicle by the exuding therefrom of a plastic liquid or lymph; this latter gradually becomes granular, and is then formed into cells, which, as the growth proceeds, are elongated into fibres, which form the central portion of the hair.
In consistence they are firm and elastic, or fluctuating, and are incorporated with the overlying skin, but movable on the deeper structures. The orifice of the partly blocked sebaceous follicle is sometimes visible, and the contents of the cyst can be squeezed through the opening. The wall of the cyst is composed of a connective-tissue capsule lined by stratified squamous epithelium.
The hair and the nails are important modifications of the epidermis. *A Hair* is a slender cylinder, formed by the union of epidermal cells, which grows from a kind of pit in the dermis, called the hair follicle. The oval and somewhat enlarged part of the hair within the follicle is called the root, or bulb, and the uniform cylinder beyond the follicle is called the shaft.
The scale is flat and is a fold of the epidermis not arising from an invaginated follicle. The feather, on the other hand, is a tubular structure arising from a papilla at the base of a deep follicle extending inwards from the surface of the skin. As the feather grows the papilla grows with it.
I have already mentioned that this process does not occur in Teleosteans whose ovaries were studied by me. These were species of Teleosteans in which fertilisation is external. Lin. Marshall himself examined sections of the corpus luteum of Ornithorhynchus and saw much hypertrophied and apparently fully developed luteal cells, but no trace of any ingrowth from the wall of the follicle.
The true corpora lutea arise from follicles in which the ova have become mature and from which they have escaped through the surface of the ovary. Buds from the theca interna invade the follicle and form the connective tissue of the corpus luteum.
The materials for this, at any rate, passed through the follicle cells, and it is probable that these cells were not entirely passive, but actively secretory in the process. Substances diffusing from the ovum would be present in the follicle cells during this process, and probably act as a stimulus.
As the destined ovum within its nest, the follicle, grows, its fluid affects the interstitial cells to send their specific stuff into the blood. There it circulates, hits this gland and that, makes some more active, others less, transforms the chemistry of the cells, and engorges the mucous membranes, most of all those of the nose and of the uterus.
A hair is formed by a depression, or furrow, the inner walls of which consist of the infolded outer skin. This depression takes the form of a sac and is called the hair-follicle, in which the roots of the hair are embedded. At the bottom of the follicle there is an upward projection of the true skin, a papilla, which contains blood-vessels and nerves.
If this theory is sound, it would follow that corpora lutea are not formed in cases where the ova are not retained in the oviduct during their development. The essential process in the development of these structures is the hypertrophy and, in some cases at least, multiplication of the follicular cells in the ruptured follicle.
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