[ 025 ] 



Fig. 450. 



CHAPTER IV. 



THE OVA OF BIRDS. 



IT lias been stated, with regard to the ovaries of birds, that tho 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. Tt 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 orultim and accessory parts (Fig. 450). 



OVULCM. 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 cicairicula (8), a yellowish-white disc, in 

 the centre of which exists, 3, Purkinje'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 arc 

 designated vitelline globules. 

 These globules are white in 

 the centre of tho egg, and have 

 only a few nuclei ; in the re- 

 mainder of the mass they are 

 much more voluminous, and 

 contain a large number of 

 inn i illations 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 

 liottlr IKI}" .1 liguro, named 

 tin: Intebra (9) by Purkinje, is 

 a cavity. 



ACCESSORY PAKTS.- -These 

 comprise : 1, the white or 

 albumen (3), disposed in three 

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

 time different periods during its course along tho oviduct; i?, The 

 i-JmlazcK (6), species of albuminous ligaments twisted in a spiral manner, 

 and attaching tho yolk to the testaceous membrane ; 8, T> 

 *1i ill-membrane (4), which offers towards the obtuse polo a doubling into 

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

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



The testaceous or shell membra, <> \^ composed of a closely woven 

 tino; it owes its opacity to tho air it contains in iN m.-ln <. 



l. 



SECTIONAL VIKW OF FOWL*> 



Yellow yolk composed of successive layers : _'. 

 Vitelline membrane ; 3, Layers of albumen ; 4. 

 Two principal layers of the lining membrane of 

 the shell ; 5, Calcareous shell ; 6, Chalaza- ; 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 

 '.ivity, or latebra, 9. 



