CHAPTER VI. 



THE SKIN AND ITS APPENDAGES. 



The Skin. — Considerable assistance in the understanding of the 

 diseases to which the skin is liable and of the nature of the growths 

 which arise from the epidermis, such as corns, bunions and cancer, 

 is to be obtained by studying the manner in which the skin is 

 developed. At first (see Fig. 51) the human embryo is covered 

 by a single layer of epithelium (epiblast or ectoderm) as is the case 

 in the adult amphioxus. By the end of the 1st month there are 

 two layers, the lower representing the germinal layer ; the upper, 

 the corneous layer (Pig. 52). In the 4th month intermediate 

 layers appear, from which the stratum mucosum and the stratum 

 lucidum are differentiated (Pig. 53). 



.„,-i,„j *hs==^-^ .corneous layer ___ 



^epiblast /^^a^ . , , 3 corneouslauer 



'^nnhlnit ^^^^^-germinal layer Ifratumlucim 



mesoblast ^mmm^mesoblast ^Mlay^T 



Fin. 51. Pro. 52. Fig. 53. 



Fig. 51.— The strata of the skin during the first month. 

 Fig. 52.— The strata of the skin during the second month. 

 Fig. 53. — The strata of the skin from the sixth month onwards. 



The epidermis rests at first on undifferentiated mesoblast or 

 mesoderm, consisting of small round cells closely imbedded in a 

 mucoid matrix. This is the normal structure of undifferentiated 

 mesoblast. The superficial mesoblastic cells are subsequently 

 condensed beneath the epidermis to form a corium. They become 

 fibrillated and by the fifth month the mucoid substance has almost 

 disappeared, but even in adult life, when the thyroid is diseased 

 or removed, a mucoid substance may reappear, and a condition 



