CHAPTEE IV. 



THE OVA OF BIEDS. 



It has been, stated, with regard to the ovaries of birds, that the ovules 

 compose nearly the whole mass of these organs. When it leaves the ovary, 

 the ovum presents the same parts as that of mammals, only differing in 

 volume, which is enormous in birds. It is in passing through the oviduct 

 that it is covered with a thick layer of albumen and enveloped in a white 

 membrane, and then a shell, to compose what is usually known as an egg. 

 It is therefore composed of the ovulum and accessory 'parts (Fig. 450). 



OvuLUM. — There is found in the ovulum : 1, A vitelline membrane (2), 

 thinner, proportionately, than that of mammals ; it is fibrous, and shows on 

 a certain point of its surface : 2, The cicatricula (8), a yellowish-white disc, in 

 the centre of which exists, 3, PurJcinje's vesicle, or the germinal vesicle ; the 

 yolk or vitellus (1), which fills the enveloping membrane. In the mass of the 

 vitellus is seen what appears to be a bottle-shaped cavity, the long neck of 

 which is applied to the cicatricula. 



The vitellus of the egg of birds differs from that of mammals in its 

 anatomical composition, being „. 



entirely composed of what are '^' 



designated vitelline globules. 

 These globules are white in 

 the centre of the egg, and have 

 only a few nuclei ; iu the re- 

 mainder of the mass they are 

 much more voluminous, and 

 contain a large number of 

 granulations which give them 

 their yellow colour. It is the 

 presence of the clear vitelline 

 nucleated globules in the centre 

 of the egg, which has given 

 rise to the surmise that the 

 bottle-shaped figure, named 

 the latebra (9) by Purkinje, is 

 a cavity. 



AccEssoEY Parts. — These 

 comprise: 1, the white or 

 albumen (3), disposed in three 

 layers of different densities, and which are deposited around the yolk at 

 three different periods during its course along the oviduct; 2, The 

 chalazce (&), species of albuminous ligaments twisted ma spiral manner, 

 and attaching the yolk to the testaceous membrane ; 3, The testaceous or 

 shell-membrane (4), which offers towards the obtuse pole a doublmg into 

 two layers, between which are found : 4, The air-chamber (7), so named 

 from the air it contains ; 5, The shell (5), decomposable into several layers. 



The testaceous or shell membrane is composed of a closely woven fibroid 

 tissue • it owes its opacity to the air it contains in its meshes. 



SECTIONAL VIEW OF FOWL'S EGG. 



Yellow yolk composed of successive layers ; 2, 

 Vitelline membrane; 3, Layers of albumen; 4, 

 Two principal layers of the lining membrane of 

 the shell ; 5, Calcareous shell ; 6, Chalazaj ; 7, Air- 

 space between the two layers of the shell ; 8, 

 Cicatricula, with its nucleus, beneath which is 

 seen the canal leading down to the white yolk 

 cavity, or latebra, 9. 



