THE EGG. 229 



one, which alone is functional, forms in the adult hen a wide 

 convoluted tube, which commences in front with a long, oblique, 

 funnel-like mouth, bordered by a fimbriated edge, and lying in 

 close contact with the ovary. Behind this mouth comes a long, 

 convoluted, but thin-walled part of the oviduct, and then a 

 short terminal part with very thick walls, which opens into the 

 cloaca, and through this to the exterior. 



The ovum at the time of its discharge from the ovary 

 consists of the yolk alone, inclosed in the vitelline membrane. 

 The albuminous investment, or ' white of the egg,' is formed 

 around the yolk by the walls of the first, or thin-walled, part of the 

 oviduct ; and the shell membrane and egg-shell are added while 

 the egg is in the thick-walled terminal part of the oviduct, just 

 before being laid. 



The ovaries can be recognised in chick embryos during the 

 third day of incubation, as a pair of slightly modified tracts of 

 the peritoneal epithelium which clothes the dorsal wall of the 

 body cavity, close to the root of the mesentery. This germinal 

 epithelium is at first merely a longitudinal strip of peritoneum, 

 of which the component cells are columnar instead of squamous 

 in shape. By multiplication of the cells, to form a layer several 

 cells thick, the strip becomes a prominent ridge. Vascular 

 connective tissue soon grows in along the axis of this genital 

 ridge, and renders it still more conspicuous. 



Almost from the first, certain of the epithelial cells of the 

 genital ridge differ from their fellows in their greater size and 

 more spherical shape, and in possessing nuclei of unusual dimen- 

 sions ; these larger cells are the primitive ova or gonoblasts. 

 The primitive ova rapidly increase in size, and move from the 

 surface, where they all take their origin, into the deeper parts of 

 the genital ridge ; the smaller, indifferent epithelial cells at the 

 same time becoming arranged so as to form follicles around them. 



The follicular epithelial cells serve to nourish the ova, 

 drawing nutriment from the blood-vessels of the genital ridge, 

 and passing it on, probably after elaborating it, into the ovum. 

 Within the ovum the food matter undergoes further changes, and 

 is deposited in the form of granules, from which the definite yolk- 

 granules of the fuUy-formed egg are finally derived. 



During these changes the nucleus of the primitive ovum 

 increases greatly in size, and acquires a distinctly vesicular 



