294 HISTOGENESIS 



stratum germinativum (Fig. 299 B). After the fourth month the epider- 

 mis becomes many layered. The inner layers of cells now form the stra- 

 tum germinativum and are actively dividing cells,' united with each other 

 by cytoplasmic bridges. The outer layers of cells become cornified, the 

 cornification of the cells proceeding from the stratum germinativum 

 toward the surface. Thus, next the germinal layer, are cells containing 

 keratohyalin which constitute the double-layered stratum granulosum. A 

 thicker layer, above the stratum granulosum, shows cells in which drops 

 of a substance called eleidin are formed. These droplets, supposed to 

 represent softened keratohyalin, give these, cells a clear appearance when 

 examined unstained. Hence the layer is termed the stratum lucidum. 

 In the outer layers of the epidermis the thickened walls of the cells become 

 cornified and in the cells themselves a. fatty substance collects. These 

 layers of cells constitute the stratum corneum. Its cells are also greatly 

 flattened, especially at the surface. , . 



When the hairs develop they do not penetrate the outer periderm layer 

 of the epidermis, but, as they grow out, lift it off (sixth month). Hence 

 this layer is known also as the epitrichium (layer upon the hair). Des- 

 quamated epitrichial and epidermal cells mix with the secretion of the 

 sebaceous glands to form the pasty vernix caseosa that smears the fetal 

 skin. Pigment granules appear soon after birth in the cells of the stratum 

 germinativum. These granules are probably formed in situ. Negro chil- 

 dren are quite light in color at birth, but within six weeks their integument 

 has reached the definitive degree of pigmentation. 



The derma, or corium, of the integument is developed from mesen- 

 chyme, perhaps from definite dermatomes (Fig. 323) of the mesodermal 

 segments (p. 292). At about the end of the third month a differentiation 

 into the compact corium proper and the areolar subcutaneous tissue occurs. 

 From the corium, papilla project into the stratum germinativum. 



Anomalies. The deposition of pigment in ''the epidermis and elsewhere may fail 

 (albinism), or be over abundant (melanism). The defects of pigmentation sometimes 

 affect local areas only. Naevi are either pigmented spots ('moles'), or purple discolora- 

 tions ('birthmarks') caused by cavernous vascular plexuses in the corium. Ichthyosis 

 results from an excessive thickening of the stratum corneum. In severe cases, horny 

 plates 5 mm. thick are formed; these are separated by deep cracks. Dermoid cysts (p. 

 218) resulting from epidermal inclusions, are not infrequent along the lines of fusion of 

 embryonic structures, e. g., branchial grooves, mid-dorsal and mid-ventral body wall. 



THE HAIR 



Hairs are derived from thickenings of the epidermis and begin to 

 develop at the end of the second month on the eyebrows, upper lip, and 

 chin. The hair of the general body integument appears at the beginning 

 of the fourth month. 



