FEMALE OKGANS OE BIRDS. 247 



is shown in focus at d : in E, ov is the ovisac mth its epithelial 

 lining, v the granular yolk surrounding, ^ the germinal vesicle 

 or developed * germ-cell.' F is part of an ovule of -^-^ of an inch 

 in diameter, highly magnified: v, minutely granular or primitive 

 yolk-substance; ^, germinal vesicle ; 2-, ' thick consolidated mem- 

 branous layer which formed a vesicular covering for the primitive 

 ovule, and which corresponds to the zona pellucida of the mam- 

 miferous ovum.' ^ 



In G and H, Prof. Allen Thomson gives diagrammatic figures 

 of the earliest stages of formation of the ovarian ovum in a Black- 

 bird : figs. I and K ' are intended to illustrate, diagrammatically, 

 the view, that after the disappearance of the zona, and the for- 

 mation of the larger granular yolk-cells, the outer layer of the 

 cells of this substance forms the permanent vitelline membrane of 

 the bird's egg ; vd, * remains of minutely granular yolk, forming 

 the vitelline disc round the germinal vesicle ; s^, large corpuscles 

 of the yolk ; vrrif outer layer of the cells of the same, on which 

 the vitelline membrane is afterwards formed.' ^ 



The germinal vesicle, with the firmer primitive vitelline granules 

 (J germ-yolk,' K, vd), moves from the centre to the periphery of 

 the ovum, which then begins to expand by the addition of the 

 softer ^ food-yolk,' ib. sff : this seems to be due to cells throAvn off 

 by, and to fluid exuding from, the inner surface of the ovisac, ov, 

 the cells greatly and rapidly increasing in number and acquiring 

 the characteristic yellow or orange colour of the yolk in birds. 



At the earlier stages of the developement of the ova the ovarium 

 appears as a flattened solid, granular body, attached by a fold of 

 peritoneum, or of air-cell, to the bodies of the middle dorsal 

 vertebras, fig. 125, a. 



At first, the right and left ovaria are similar in size, fig. 127, c: 

 but the symmetry is soon disturbed by concentration of de- 

 velopement in the left ovarium (fig. 125, a), the right one, «', 

 remaining stationary and ultimately, in most birds, disappearing. 



The enlaro-ement of the ovarian ovum is now due to the accu- 

 mulation of the yellow or ^food ' yolk, with concomitant distension 

 of the membrana vitelli and of the ovarian capsule, or ' calyx,' 

 fig. 126, a, d, which maintains its connection Avith the rest of the 

 ovarium by a contracted base or pedicle. 



The calyx consists of two membranes, united together by lax 

 tissue and blood-vessels : these ramify as in fig. 126, c, converging 

 toward a white transverse line or band across the most prominent 



' cccviir. p. 76. ^ lb. 



