24 ELEMENTARY BIOLOGY. [CHAP. 



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 apertures 

 of the ureters. Remnants of the oviducts are usually pre- 

 sent in the male. 



Each ovum, when ripe, consists of a protoplasmic yolk- 

 laden mass Qi'vitellus t enclosed in a structureless vitelline 

 membrane, and containing a germinal vesicle, within which 

 are several 'germinal spots.' One half of the vitellus is 

 deeply coloured, the other pale. 



The so-called ductless glands other than the spleen 

 already described, are three in number. The Thymus ; a 

 small rounded body situated immediately behind the sus- 

 pensorium, in a position corresponding to the dorsal ends of 

 the obliterated branchial arches. The Thyroid; represented 

 by two or more oval bodies, which are found attached to 

 the bases of the aortic arches. The Adrenals ; yellow 

 bodies imbedded in the ventral face of the kidney. 



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 

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

 are found in the viscera and vessels. The former are usually 

 arranged in sets, the actions of which may be antagonistic 

 and simple or coordinated and complex. An account of 

 the disposition of the muscles in the hind-limb will be 

 found in the Laboratory work. 



The nervous system is conveniently divisible into two 

 parts, the cerebro-spinal and the sympathetic. The cerebro- 

 spinal nervous system again consists of the brain, or encepha- 



