450 



AVES. 



passes into an erectile bulb — the rudiment of a glans penis. 

 In the female generative organs the ovary and oviduct of the 

 right side are reduced or entirely absent. The generative organs 

 of the left side, however, are correspondingly larger at the breed- 

 ing season. The oviduct is much coiled, and is divided into 

 three regions : (1) The wide abdominal ostium in front ; (2) the 

 coiled glandular part which secretes, from the glands of its longi- 

 tudinally folded mucous membrane, the albumen which is added 

 in layers and is twisted together at the ends to form the chalazae ; 

 (3) a posterior short and wide portion — the so-called uterus — 

 which serves to produce the variously coloured egg-shell, and 



opens by a short 

 P^ K5 ^^^ narrow ter- 



minal region into 

 the cloaca on 

 the outer side 

 of the corres- 

 ponding ureter. 

 When there are 

 copulatory parts 

 in the male, 

 there are also 

 clitoris-like struc- 

 tures a t the 

 same place in 

 the female. 



Nothing seems to be known as to the actual process of copu- 

 lation in birds. 



Development. — Birds are, without exception, oviparous. The 

 egg is remarkable for the large amount of yolk (distinguishable 

 into white and yellow yolk), and its porous calcareous shell (Fig. 

 246). The development requires a high temperature, at least 

 equal to that of the blood. The necessary heat is usually sup- 

 plied by the bird during incubation. 



Fertilization takes place in the upper region of the oviduct 

 before the secretion of the albumen and of the shell membrane, 

 and is at once followed bj^ the partial (discoidal) segmentation 

 (Fig. 247) which only implicates the clear part of the yolk (for- 

 mative yolk) around the germinal vesicle — the germinal disc or 

 so-called tread of the cock (cicatricula). 



Fig. 246. — Diagrammatic longitudinal section through an un- 

 developed hen'a egg (after Allen Thomson). Bl germinal 

 disc ; Ch chalazae ; DM vitelline membrane ; EW albumen ; 

 OD yellow yolk ; KS calcareous shell : LR air-chamber ; S shell 

 membrane; WD white volk. 



