852 



STRUCTURE OF A HEN'S EGG. 



characteristic white velk elements, processes pass into the yellow yelk. A part passes as an 

 exceedingly thin layer round the yelk, or cortical protoplasm. [The cicatncula in an umncubated 

 eat is always uppermost whatever the position of the egg, provided the contents can rotate 

 freelv and this is due to the lighter specific gravity of that part of the yelk in connection with 

 the cicatricula. In a fecundated egg the cicatricula has a white margin (the area opaca), sur- 

 roundings clear transparent area, the beginning of the area pellucida, containing an opaque 



terminal Blastoderm. 



reside 



.m.l l>ot. 



Its processes. 



Marginal 

 protoplasm. 



Vitelline 

 membrane. 



Fig. 642. Fig. 643. 



Scheme of a meroblastic egg. a, White ; b, yellow yelk granules, 



spot in its centre. If an egg be boiled very hard and a section made of the yelk, it will be found 

 to consist of alternating layers of white and yellow yelk. The outermost layer is a thin layer 

 of white yelk, which is slightly thicker at the margin of the cicatricula. "Within the centre of 

 the yelk is a flask-shaped mass of white yelk, the neck of the flask being connected with the 

 white yelk outside. This flask-shaped mass does not become so hard on being boiled, and its 

 upper expanded end is known as the "nucleus of Pander." The great mass of the yelk is 

 made up, however, of yellow yelk.] Microscopically, the yellow yelk consists of soft yellow 

 splu-res, of from 23-100 /u, in diameter, and they are often polyhedral from mutual pressure (fig. 

 643, b). [They are very delicate and non-nucleated, but filled with fine granules, which are, per- 

 haps, proteid in their nature, as they are insoluble in ether and alcohol. They are developed by 

 the proliferation of the granulosa cells of the Graafian follicle, which also seem ultimately to 

 form the granulo-fibrous double envelope or the vitelline membrane (Eimcr). The whole yelk 

 of the lien's egg is regarded by some observers as equivalent to the mammalian ovum plus the 

 tri) corpus luteum. Microscopically, the white 



yelk consists of small vesicles (5-75 /u,) con- 

 taining a refractive substance and larger 

 spheres containing several smaller spherules 

 (fig. 643, a). The whole yelk is enveloped by 

 the vitelline membrane, which is transparent, 

 but possesses a fine fibrous structure, and it 

 seems to be allied to elastic tissue.] 



When the yelk is fully developed within the 

 Graafian follicles of the hen's ovarium, the 

 follicle bursts and discharges the yelk, which 

 passes into the oviduct, where in its passage it 

 rotates, owing to the direction of the folds of 

 the mucous membrane of the oviduct. The 

 numerous glands of the oviduct secrete the 

 albumin, or white of the egg, which is deposited 

 in layers around the yelk in its passage along 

 the duct, and forms at the anterior and posterior 

 Fowl's eg* after thirty hours incubation, a, chalaz ae [The chalazae are two twisted cords 

 shell ; n>, shell-membrane ; b', air-chamber- com Posed of twisted layers of the outer denser 

 c, boundary between outer and middle portion ^ of , S e all ? umm ; ^ e * teud from the e 

 of albumin; d, more fluid albumin; - If ^ the yelknot ^itetotte outer part of 

 chalazae ; v, yelk ; av, area opaca ao area albumin.] [The albumin is invested by the 

 vasculosa, and in its centre is the embrVo ^ enibrana testacea, or shell-membrane, which 

 , . .. . . J ' is composed of two layers an outer thicker 



and an inner thinner one (fig. 644). Over the greater part of the albumin these two layers 

 are united, but at the broad end of the hen's egg they tend to separate, and air passing through 



Fig. 644. 

 after thirty hours incubation. 



