56 



COMPARATIVE PHYSIOLOGY, 



cordance with the diverse circumstances under which ova ma- 

 ture and develop, certain variations in structure, mostly of the 

 nature of additions, present themselves. 



Thus, ova may be naked, or provided with one or more cover- 

 ings. In vertebrates there are usually two membranes around 

 the protoplasm of the ovum : a delicate covering {Vitelline 

 membrane) beneath which there is another, which is sieve-like 

 from numerous perforations {zona radiata, or z. pellucida). 

 The egg membrane may be impregnated with lime salts {shell). 

 Between the membranes and the yelk there is a fluid albumi- 

 nous substance secreted by the glands of the oviduct, or by other 

 special glands, which provide proteid nutriment in different 

 physical condition from that of the yelk. 



The general naked-eye appearances of the ovum may be 

 learned from the examination of a hen's Q.gg, which is one of 



my 

 w.y. \ 



U. 



ck.i 



FiQ. 56. — Diagrammatic section of an unimpregnated fowl's egg (Poster and Balfonr, 

 after Allen Thomson). 6?, blastoderm or cicatricula; w. v, white yelk; y. y, yel- 

 low yelk; cli. /, chalaza; i. s. m, inner layer of shell membrane; 8. m, outer layer 

 of shell membrane; », shell; a. c. h, air-space: w, the white of the egg; v. I, vitel- 

 line membrane ; x, the denser albuminous layer lying next the vitelline mem- 

 brane. 



the most complicated known, inasmuch as it is adapted for 

 development outside of the body of the mother, and must, con- 

 sequently, be capable of preserving its form and essential vital 

 properties in a medium in which it is liable to undergo loss of 

 water, protected as it now is with shell, etc., but which, at the 



