THE OVUM. 



949 



material penetrates into the originally small and simple ovular cell and causes it 

 to swell considerably. The embryology of the bird's egg has shown that only 

 the small, round, white protoplasmic germinal layer at the center of the surface 

 of the yolk (cock's treadle, cicatricula), from 2.5 to 3.5 mm. wide and from 0.28 

 to 0.37 mm. thick, corresponds to the contents of the mammalian ovum and is 

 therefore the formative yolk. It contains the germinal vesicle and the germinal 

 spot (Fig. 359). From this, which contains also the characteristic white yolk- 

 elements (Fig. 360, a) processes extend into the yellow yolk (Fig. 359). In 

 addition, a flask-shaped mass of white yolk extends into the center of the yellow 

 yolk (Purkinje's latebra) ; the yolk is surrounded by an extremely thin mem- 

 brane (white yolk-membrane or "the cortical protoplasm) (Fig. 359 and Fig. 360) 

 The yellow yolk (nutritive yolk) consists of soft, yellow non-nucleated cellular 

 structures from 23 to 100 , in diameter, and somewhat polyhedral in shape from 



IL 



W 



FIG. 358. I. Ovarian Tube (from a Newborn Child) in Process of Follicle-formation: a a, Ova in the midst of 

 the epithelial cells of the surface of the ovary; b, ovarian tube with ova and epithelial cells; c, a constricted-pff 

 small follicle, with ovum. //, Open ovarian tube of a bitch six months old. ///, Isolated human primordial 

 ovum. IV, Older follicle with two ova (o 6) and the cells of the granulosa (g) (dog). V, Portion of the sur- 

 face of a mature rabbit's ovum; z, zona pellucida; d, yolk; e, adherent cells of the granulosa (after Waldeyer). 

 VI, Expulsion of the first polar body. VII, Expulsion of two polar bodies (after Fol). 



mutual pressure (Fig. 360, 6). These result from proliferating hyperplasia of the 

 granulosa-cells of the Graafian follicle, which finally give rise also to the granulo- 

 fibrous two-layered yolk-membrane (Fig. 359). The entire yolk of the bird's egg 

 has been considered equivalent to the mammalian ovum, together with its cor- 

 pus luteum. 



When the yolk-globule in the bird's ovary is fully developed, the capsule of 

 the Graafian follicle is ruptured and the yolk-globule passes in a rotatory fashion 

 through the oviduct, the folds of whose mucous membrane, like the riflings of a 

 gun-barrel, always cause the rotation to take place in a definite manner. Numer- 

 ous glands in the oviduct secrete the albumin in which the yolk is enveloped in 

 layers, the chalazae being formed at either pole. As the tenacious layers of albu- 

 min tend to unroll again, the albuminous layer is rotated about the yolk in the 

 bird's egg, and if freshly laid eggs are permitted to float in concentrated solu- 

 tion of sodium chlprid, all will rotate in the same direction. 



The albumin in the eggs of nesting birds is vitreous and translucent when 



