452 TEXT-BOOK OF EMBRYOLOGY. 



Occasionally pigment develops in excess over larger or smaller areas of the 

 skin, giving rise to the so-called ncevi pigmentosi. In some cases, on the other 

 hand, there is total or almost total lack of pigment in the skin and hair (usually 

 accompanied by defective pigmentation of the iris, chorioid and retina) 

 a condition known as albinism. There are also instances of partial albinism. 

 The influence of heredity in albinism is doubtful, for albinos are usually the 

 children of ordinary parents. 



The angiomata (lymphangiomata, haemangiomata) found in the skin are due 

 to dilated lymphatic or blood channels, the color in haemangiomata being due 

 to the haemoglobin in the blood. 



Dermoid Cysts. The congenital dermoid cysts not infrequently found in or 

 under the skin are usually situated in or near the line of fusion of embryonic 

 structures, as in the region of the branchial arches, along the ventral body 

 wall and on the back. During the fusion of adjacent structures, portions of the 

 epidermis become constricted from the parent tissue and come to lie in the der- 

 mis, where they continue to grow and produce cystic masses and sometimes 

 give rise to hairs and sebaceous glands. This type of dermoid is to be dis- 

 tinguished from that found for example in the ovary, in which derivatives of 

 all three germ layers are present (see Chap. XIX). 



ANOMALIES OF THE EPIDERMAL DERIVATIVES. Occasionally hair develops 

 in profusion over areas of the skin that naturally possess only a fine, silky growth, 

 such, for example, as a woman's face. Or nearly the entire body may be 

 covered by an unusual amount of hair. Such conditions known as hyper- 

 trichosis possibly represent the persistence and continued growth of the 

 lanugo (p. 447) and in this sense are to be regarded as the result of arrested 

 development (Unna, Brandt). Congenital absence of the hair (hypotrichosis, 

 alopecia) is a rare anomaly and is usually accompanied by defective develop- 

 ment of the teeth and nails. 



Sebaceous cysts, generally regarded as due to accumulation of secretion 

 in the sebaceous glands, sometimes probably represent remnants of displaced 

 pieces of epidermis apart from the hairs (Chiari) . 



Supernumerary mammary glands (hypermastid) and nipples (hyperthelia) are 

 not infrequently present in both males and females. They are usually situated 

 below the normal mammae (rarely in the axillary region), in a line drawn from 

 the axilla to the groin, and probably represent persistent and abnormally de- 

 veloped portions of the milk ridge (see p. 449) In very rare cases a super- 

 numerary gland develops in some other region (even on the thigh) . If the 

 mammary glands are morphologically allied to the sweat glands (p. 450), these 

 misplaced mammae are suggestive of anomalous development of some of the 

 sweat gland anlagen. 



