176 ELEMENTARY BIOLOGY. [XIIL 



mucli folded and plaited in the breeding season. The in- 

 terior of each is hollow, and is divided into several chambers. 

 •Innumerable ovisacs, containing dark-coloured ova, are scat- 

 tered through the substance of the ovary and give rise to 

 projections upon the inner surface of the ovarian chamber 

 as they become fully developed. 



The oviducts are long convoluted tubes situated on each 

 side of the dorsal wall of the abdominal cavity to which they 

 are connected by peritoneal folds ; each curves over the outer 

 face of the root of the lung. Their anterior ends are very 

 slender, and terminate by open mouths at the sides of the 

 pericardium, between the attachment of the diaphragm and 

 the lobe of the liver. The fold of peritoneum which serves 

 as a ligament, holding the lobe of the liver to the dia- 

 phragm, oesophagus and posterior wall of the pericardium, 

 in fact constitutes the outer lip of the oviducg,! aperture. 

 For the greater part of their length their walls are thick and 

 glandular, and swell up when placed in water. Posteriorly, 

 the oviducts dilate into capacious thin-walled chambers and 

 end, close together, by openings which are situated in the 

 dorsal wall of the cloaca immediately in front of the aper- 

 tures of the ureters. 



Each ovum, when ripe, consists of a structureless vitelline 

 membrane, inclosing a vitellus, within which is a germinal 

 vesicle, containing several ' germinal spots.' One half of the 

 vitellus is deeply coloured, the other pale. 



The actions of the different parts of the organism of the 

 Frog are coordinated with one another and brought into 

 relation with the external world by means of the muscular 

 and nervous systems and the organs of sense. 



The muscles consist partly of striped and partly of un- 

 striped fibres, the former being confined to the muscles of 

 tlie head, trunk and limbs and the heart, while the latter 



