252 



ANATOMY OF VERTEBRATES, 



yolk' over the ^germ-yolk.' In fig. 129, B is an enlarged view 

 of the cicatricula as seen from above on the surface of the yolk 

 in an impregnated egg : the dark central space is the ' transparent 

 area ' surrounded by the ^ opaque area,' and by one or two de- 

 licate * halones.' C is the cicatricula of an unfecundated laid 

 eo-o" : instead of the central transparent area a number of rather 

 irregular transparent spots are seen. 



The yolk forms an ellipsoid mass, somewhat flattened on the 

 cicatricular surface, and consists of the external coloured part, 

 fig. 130, B, d, in concentric layers indicative of successive deposit, 

 and of a central lighter-coloured part, ib. c, about one-fourth of 



Fowl's egg and irtructurc of tlie yolk as exhibited by a section, cccviii. 



the diameter of the whole. The margins of yolk-layers, inter- 

 rupted by the ^ cicatricula' and its canal, may form the ' halones.' 

 The yolk-layers, dj d, usually show some diversity of tint. 



The ripe ovarian ovum, having passed into the oviduct, is 

 propelled by the peristaltic action of that tube in a rotatory course 

 to the ' uterus.' The contact of the membrana vitelli stimulates 

 the exudation of the product of the lining membrane in a denser 

 state than usual, which forms a kind of accessory tunic, and 

 is continued, thread-like, from near each pole of the ellipsoid, 

 usually a little toward that half which is opposite the one sup- 

 porting the cicatricula: these filaments, fig. 130, A, c, are the 

 * chalazai,' and the layer of dense albumen from which they 

 are continued is called the '^ membrana chalazifera.' During 

 the passage of the egg and its acquisition of successive deposits 

 of the ordinary albuminous secretion, the chalazoe become twisted 

 in opposite directions, fig. 131, B, and ultimately the one next 

 the small end of the egg contracts some adhesion to the membrane 

 linin.o: the shell there. In fig. 131, A shows the ovum from the 



