AVES 



315 



The "egg" of the bird (Fig. 160) is a large and complex structure, 

 consisting of the ovum proper, the albuminous layers, shell mem- 

 branes and shell. The ovum, or what is usually referred to as the 

 yolk, is a single food-gorged cell inclosed within a vitelline membrane 

 and with a single nucleus or germinal vesicle. The active protoplasm 

 of the ovum is largely aggregated in a small region situated at the 

 animal pole of the cell, called the germinal spot, where lies the nucleus. 



Ml. 



p'y.^. 



Fig. 160. — Diagram of hen's egg to show envelopes, and general relations of 

 parts, A. C, air chamber; Alh, albumen; Bl, blastoderm; dial, Chalaza; /. S. M, 

 inner layer of shell membrane; L, lateljra; NL, neck of latebra; A^. P, nucleus of 

 Pander; O.S. M, outer shell membrane; ]>. r. s, perivitelline space; .s, shell; B. M, 

 vitelline membrane; W. Y, white yolk; 1', Y, yellow yolk. (From Lillie's " Develop- 

 ment of the Chick " [Henry Holt and Company].) 



This small mass of hyaline protoplasm is continuous with a thin 

 sheath of protoplasm that surrounds and incloses the entire yolk 

 mass and to a certain extent permeates the body of the yolk. 



Immediately surrounding the ovum is a thick viscous layer of al- 

 bumen that is .swathed about the ovum and prolonged on opposite 

 sides into twisted ropes, called chnkvae, that fasten the ovum to the 

 shell membranes and suspend it in such a way that it cannot come 

 in contact with the shell. Between the chalazal layer of albumen and 



