308 HISTOGENESIS 



numerous pairs of mammary glands are developed along the milk line (pig, dog, etc.) ; in some a 

 pair of glands is developed in the pectoral region (primates, elephants) ; in others only in the 

 inguinal region (sheep, cow, horse). In man supernumerary mammary glands developed along 

 the milk line are of not infrequent occurrence. 



FIG. 295. Sections representing three successive stages of development of the human mammary 

 gland (Tourneux): A, fetus of 32.40 mm. (1.3 in.); B, of 10.16 cm. (4 in.); C, of 24.35 cm. (9.6 in.); 

 a, epidermis; b, aggregation of epidermal cells forming anlage of gland; c, galactophorous ducts; d, 

 groove limiting glandular area; e, great pectoral muscle; /, unstriated muscular tissue of areola; g, 

 subcutaneous adipose tissue. 



THE NAILS 



The anlages of the nails proper are derived from the epidermis and may be 

 recognized in embryos of 45 mm. A nail anlage forms on the dorsum of each 

 digit extending from the tip of the digit almost to the articulation of the terminal 

 phalanx. At the base of the anlage, that is, proximally, the epidermis is folded 

 inward to form the proximal nail fold (posterior nail fold of the adult). This is 

 curved, convex proximally, and extends transversely to the dorsum of the digit 

 (Fig. 296 A, C). The nail fold also extends laterally on either side of the nail 

 anlage and forms the lateral nail fold of the adult (A, B). 



The matrix of the nail is developed in the proximal nail fold (C) . In a layer 

 of epidermal cells, lying parallel to the dorsum of the digit, there are developed 

 keratin or horn fibrils during the fifth month of fetal life. These appear without 

 the previous formation of keratohyalin granules as is the case in the cornification 

 of the stratum corneum. The cells flatten and form the plate-like structure of 

 which the solid substance of the nail is composed. Thus the nail substance is 

 formed in the proximal nail fold. Over the area termed the lunula (the whitish 

 crescent at the base of the adult nail) the nail is pushed toward the tip of the digit 

 by the development of new nail substance in the region of the nail fold. The nail 

 matrix, according to Bowen, represents a modified stratum lucidum of the epider- 

 mis. The stratum corneum of the epidermis for a time completely covers the nail 



