515 



CHAPTER IX. 



TEGUMENTARY SYSTEM OF II^MATOCRYA. 



§ 98. Composition of Tegument. — The tegumentary organs of 

 Vertebrates, where they do not happen, as in exceptional instances 

 or parts of the body, to blend with the periosteum of the endo- 

 skeleton, are denned from subjacent structures by loose or yielding 

 connective tissue : hence the facility with which Vertebrates of 

 all classes can be ' skinned.' The part so removed is the ' tegu- 

 ment,' and constitutes the outermost of the organs differentiated 

 in the course of embryonal developement from what has been 

 termed the ' serous' or ' animal ' layer of the blastoderm. 



Tegument mainly consists of an outer epithelial layer, called 

 e epiderm,' and an inner fibrous or areolar layer, called ' derm.' 

 The tissue of the derm includes 'white fibres' and 'yellow fibres.' 

 The white element forms bands of unequal thickness, striated 

 longitudinally, but irregularly, and breaking up into fibrils of 

 different width, the finest being too minute for micrometry. The 

 white bands interlace in various directions, with a wavy course, 

 frequently subdividing, and joining those near them. The yellow 

 fibres are solitary, very elastic, disposed to curl, branching at 

 intervals of variable length, and the branches, usually as large as 

 the trunk, uniting with contiguous ones. A drop of acetic acid, 

 which instantly swells the white bands and makes them trans- 

 parent, produces no change on the yellow filaments. 1 Into the 

 derm enter bloodvessels, absorbents, and nerves : it never contains 

 fat. Epiderm consists of epithelial cells of every form — caudate, 

 tessellate, rarely ciliate — and in all stages of developement, 

 increasing in density, horizontality, and overlappingness as they 

 approach the outer surface of the skin, and blended with pigment- 

 cells and pigment-particles in proportion as they are near the derm : 

 but many other parts are specialised in the tegumentary area of the 

 blastoderm than those which, from their greater abundance and 

 constancy, give the character to the two best defined layers. 



Bulb3 or pulps of hairs and feathers, bony scutes, and fish-scales 



1 ccxc. i. p. 491. 

 VOL. I. N N 



