74 THE STRUCTURE OF THE FOWL 



intervening fluid. The granules of the white 

 yolk are exceedingly fine, those of the yellow 

 yolk being coarser. 



In whatever position the egg be placed, 

 the upper surface of the yolk is occupied by 

 a small, circular white patch, the blastoderm 

 (cicatrix or " tread "). This consists of a disc 

 of cells from which, and from which alone, 

 the chick develops. The yolk and the blasto- 

 derm are the only parts of the egg which have 

 their origin in the ovary. The albumen, shell- 

 membrane, and shell, which form more than 

 half of the total weight of a fully formed and 

 normal egg, are produced by the oviduct. 

 The importance of the oviduct is thus manifest. 



The oviduct (Figs. 12 and 39) is a convoluted 

 tube possessing considerable capacity for dilata- 

 tion. It is suspended between the two layers 

 of a fold of peritoneum which forms the mem- 

 branous dorsal and ventral ligaments of the ovi- 

 duct. The dorsal ligament has one of its borders 

 attached to the duct while the other is fixed i 

 to the dorsal wall of the body from the caudal 

 end of the body-cavity to the fourth thoracic 

 rib. The ventral ligament has a free ventral 

 border wliich is thick and muscular. 



The oviduct may be divided into five parts, 



