522 EMBRYOLOGY. 



different kinds, which are developed in different ways in the separate 

 classes of Vertebrates and which preeminently determine the external 

 appearance of the animals. 



As external processes arise the dermal teeth, and scales, the 

 feathers, hair, and nails. As invaginations of the epidermis are 

 developed the sweat-, sebaceous, and milk-glands. We will begin 

 with the former, and. not to go too far into details, will limit our- 

 selves to the organs of the skin in Mammals. 



(b) The Hair. 



The most characteristic epidermoidal structures of Mammals and 

 Man are the hairs. One can distinguish two modifications in the 

 method of their development. The ordinary method of development 

 is that which is known in Man. In this case, at the end of the 

 third embryonic month, the mucous layer grows at certain places 

 and forms small solid plugs, the hair-germs, which sink into the 

 underlying corium (fig. 292 B hk}. By afterwards elongating and 

 becoming thickened at the deep end they assume the shape of a 

 flask. Then there ensues a process similar to that which takes place 

 upon the formation of the teeth. At the bottom of the epithelial 

 plug the adjacent corium grows and forms a richly cellular nodule 

 (pa), which grows into the epithelial tissue and is the fundament of 

 the connective-tissue hair-papilla, which is early provided with loops 

 of blood-vessels. Around the whole ingrowing germ of the hair the 

 surrounding parts of the corium are afterwards more and more 

 distinctly arranged into special courses of fibres some of which run 

 lengthwise, others in a circular manner and constitute a special, 

 vascular, nutritive envelope, the hair-follicle (fig. 292 C, D, hb). 



A somewhat different method of hair-formation has been observed 

 by REISSNER, GOETTE, and FEIERTAG in certain Mammals. 



In these the first impulse to the formation of the fundament of a 

 hair is produced by a limited cell-growth of the corium immediately 

 below the epidermis. It produces a small elevation (fig. 292 A), 

 which is simply the hair-papilla itself, projecting into the epidermis. 

 Then the papilla is forced farther and farther away from the surface 

 of the skin by the growth of the epidermal cells that cover it, and 

 at last is found far removed from its place of origin and at the deep, 

 somewhat thickened end of a long epithelial plug. 



The final result is therefore the same in both cases, only the time 

 of the formation of the first fundament of the papilla and of the 



