GLOSSARY. 375 



Vestibule. (1) Part of the vertebrate organ of hearing ; (2) That part of 

 the female urinogenital organs of the rabbit which lies just within the 

 vulva ; (3) That part of the urinogenital organs of the snail which lies 

 just within the genital aperture ; (4) The ciliated funnel in vorticella 

 and paramoacium. 



Villus A hair-like projection, multitudes of which give the small intestine 

 of the rabbit a shaggy appearance. Villi are generally developed so as 

 to give additional surface, absorptive (intestine) or vascular (placenta). 



Yolk. Yelk or vitellus. The egg of the fowl consists of (1) an external 

 shell ; (2) An outer shell-membrane ; (3) An inner shell-membrane ; (4) 

 The albumen or white of egg ; (5) The vitelline membrane surrounding 

 (6) the vitellus or yolk, in one special area of which the cicatricula or 

 blastoderm, the embryo, is developed. The great mass of the yolk is 

 composed of yellow yolk, consisting of minute yolk spheres filled with 

 numerous highly refractive granules. Besides the yellow yolk there 

 are concentric layers and an internal flask-shaped mass of white yolk. 

 The term yolk is therefore applied (1) to the vitelline mass in which 

 the embryo is formed, (2) to the yolk spheres, which are to be regarded 

 as stored food-stuff for the embryo during growth within the egg. 



