lEgg in Birds ^ and the nature and seat of the Cohmr. 171 



variety. A similar structure was readily detected in other eggs, 

 as those of the common fowl, bantam fowl, thrush, hedge-spar- 

 row, &c. Careful examination of this superficial membrane led 

 to the discovery of a finer, almost structureless, but membranous 

 film beneath it. There had thus been detected a layer not 

 alluded to by Purkinje, and in addition it was found that, con- 

 trary to the statement of that observer, the outermost covering 

 of the shell really has a peculiar structure. In a word, the shell 

 of the eg^ has the same general arrangement of parts as a mu- 

 cous membrane or the skin : a superficial layer composed of cells, 

 and another generally of finer texture on which it rests; the 

 former corresponding to the epithelium, the latter to the base- 

 ment membrane, as they are denominated by histologists. 



In some cases the superficial layers are so delicate that the 

 action even of very weak acid tears them into shreds, and when 

 they are colourless or nearly so they may escape observation 

 altogether. 



It may not be irrelevant to state generally the facts ascertained 

 regarding tissues similar to those under consideration. The 

 epidermis or cuticle which covers the external surface of the 

 body is composed of layers of cells, the most superficial con- 

 tinually wearing away or falling off"; the deeper layers consist 

 usually of nucleated cells j the more external do not necessarily 

 include nuclei. The corresponding layer on the free internal 

 surfaces of the body, and which is usually called epithelium, has 

 a similar structure, consisting of flattened, often polygonal cells, 

 sometimes in contact with each other, and forming consequently 

 a continuous layer, but in some cases separated by considerable 

 interstices ; this variety is usually called tessellated or pavement 

 epithelium, to distinguish it from that kind furnished with cilia. 

 Beneath the epithelium we have the fine tissue denominated 

 basement membrane. In its simplest form it is of extreme de- 

 licacy ; " in some cases, to all appearance, perfectly homogeneous, 

 presenting no trace of regular structure, appearing like a thin 

 film of coagulated gelatine." Sometimes it is distinctly granular 

 or even composed of nucleated cells. 



The texture and consequently general appearance and thick- 

 ness of the epithelium difier very much in diff'erent eggs. In that 

 of the ostrich (PI. VII. fig. 1) it is composed of contiguous thick- 

 sided polygonal cells very much resembling the horny epidermis 

 found upon the conjunctiva covering the cornea. In that of the 

 emeu there exists a structure nearly similar, only the cells are 

 smaller, and sometimes they are not contiguous but scattered. 

 The surface of the egg of the emeu is very irregular, being co- 

 vered with ridges and intervening furrows ; it is quite possible 

 that the structure of the epithelium on the ridges may differ 



12* 



