944 



SPECIAL PHYSIOLOGY. 



which gradually shrinks, appearing for a time as a cup-shaped body. 

 As the yolk descends along the oviduct, the mucous membrane of this 

 canal, which is vascular and glandular, secretes the albumen, or white ; 

 this is now added to the surface of the yolk, being deposited in spirally- 

 arranged layers, owing to the rotation of the ovum during its descent, 

 in which it is guided by numerous spiral folds of the mucous membrane. 

 The first or inner layers of the white are the densest, and at each end 



Fig. 117. 



Fig. 117. Part of the yolk of the hen's egg, supposed to he taken from the ovarium (Allen Thomson). A, 

 portion of the surface of the yolk, showing the cicatricula, vitelliue disc, or germ-yolk, with the germinal 

 vesicle still present in its centre. (Magnified 6 diameters.) B. side view of cicatricula. C, vertical sec- 

 tional plan of cicatricula ; m, vitelline membrane ; d, granular substance of dij; g, germinal vesicle. 



or pole, are denser and semi- opaque twisted portions of the white, 

 named the chalazce ; the turns of these are in opposite directions, and 

 are also produced by the spiral movements of the yolk in its descent. 



Fig. 118. 



Fig. 118. Cicatricula of the hen's egg, after it has been laid. (Allen Thomson.) A, cicatricula, or germ- 

 yolk in a fertilized egg; it shows a transparent central area, looking dark, and a few halones or haloes 

 near the circumference. B. vertical sectional plan of the cicatricula and adjacent part of yolk, in a fertil- 

 ized laid egg; a, vitelline membrane; 6, b, thick part of cicatricula or germ-yolk, with thin part or area 

 in the centre; c. group of granules, occupying the position of the former germinal vesicle, which has 

 disappeared in the progress of evolution ; d, the canal containing white yolk, leading to the latebra ; e e, 

 the yellow, or nutritive yolk. C, cicatricula of an unfertilized egg; it has no germinal vesicle, and no 

 transparent area, but only irregular blotches. 



Towards the lower part of the oviduct, the egg, now composed of the 

 yolk and white, enters a dilated portion, known as the isthmus of the 

 oviduct, which is lined by a thick mucous membrane, provided with 



