DEVELOPMENT OF SKIN AND APPENDAGES. 



1401 



FIG. 1166. 



Sections of developing 

 skin, showing earliest stages 

 in formation of hair-follicles; 

 in D epithelial cylinder is 

 invading mesoblast. X 90. 



of a deeper row of cuboid or low columnar cells, covered by a superficial sheet, 



known as the epitrichium, composed of flattened elements often lacking in definition, 



and nuclei. During the succeeding weeks the epitrichial cells become swollen and 



vesicular and differentiated from the underlying elements, which meanwhile are 



engaged in producing the epidermis. The epitrichium 



persists until the sixth month, when it becomes loosened and 



is cast off. During the third and fourth months the ectoblastic 



cells have so multiplied, that from four to five layers are 



present, those next the mesoblast being columnar and rich in 



protoplasm, while the more superficial are irregular and 



clearer. By the middle of the fifth month, by which time 



the layers have increased to almost a dozen, the outer cells 



become horny and assume the characteristics of a stratztm 



corneum, while the deepest ones represent the stratum germi- 



nativum, with an 'intervening transitional zone. About the 



sixth month desquamation of the surface cells begins, the 



discarded epitrichial and other scales mingling with the secre- 

 tion from the sebaceous glands, which meanwhile have been 



developed, as constituents of the white unctuous coating, the 



vernix caseosa (smegma embryonum), that covers the surface 



of the foetus, especially in the folds and creases. During 



the last weeks of gestation the epidermis acquires considerable 



thickness and a sharper differentiation of its component strata. 

 The connective tissue part of the skin is developed as 



a superficial condensation of the mesoblast, that during the 



first month consists of closely placed spindle cells. Coinci- 



dently with the appearance of the fibrous fibrillse, in the 



third month, differentiation takes place within the condensed 



mesoblastic tissue, which so far exists as a uniform zone, into 



a superficial and more compact layer and a deeper and 



looser one ; the former becomes the corium and the latter the tela subcutanea. 



Within the last layer soon appear larger or smaller groups of round cells in which 



oil drops, at first minute and then of increasing diameter, indicate the beginning 



of their conversion into adipose tissue. By the sixth month the panniculus adiposus 



is established. About the fifth month the line marking the junction of cuticle 



and corium becomes uneven in consequence of the development of the papillae 



and ridges of the corium and the attendant invasion of the epidermis. Certain of 



the mesoblastic cells are transformed into the component elements of the involuntary 



muscle that occurs either associated with the hair follicles as the arrectores pilorum, 



or as the more extended tracts of the dartos. 

 The Hairs. The primary development 

 of the hair begins about the end of the third 

 month of foetal life as localized proliferations 

 of the epidermis. In section these appear as 

 lenticular thickenings and on the surface as 

 slight projections. Very soon solid epithelial 

 cylinders sprout from the deeper surface of 

 these areas and invade the subjacent corium 

 to form the anlages of the hair-follicles. The 

 original uniform outline of these processes is 

 early replaced by a flask-shaped contour in 

 consequence of the enlargement of their ends 

 which in their growth surround connective 

 tissue processes to form the hair-papillfe. 



The embryonal connective tissue immediately surrounding the epidermal ingrowth 



differentiates into the fibrous sheath and the glassy membrane. 



Meanwhile and even before the formation of the papilla the epithelial contents of 



the young follicles differentiate into an axial strand of spindle cells that later undergo 



keratinization and become the hair-shaft that grows by subsequent additions 



FIG. 1167. 



Hair-follicle 



Papilla 



Developing skin, showing later stages of forma- 

 tion of hair-follicles; surrounding mesoblast is 

 forming hair-papilla and fibrous sheath of follicle. 

 X 90. 



