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. 



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

 parts. A. C, air chamber; Alb, albumen; Bl, blastoderm; Chal, Chalaza; /. S. M, 

 inner layer of shell membrane; L, latebra; NL, neck of latebra; N. P, nucleus of 

 Pander; O. S. M, outer shell membrane; p. v. s, perivitelline space; s, shell; B. M, 

 vitelline membrane; W. Y, white yolk; Y. 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 chalazce, 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 



