252 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



a delicate vitelline membrane, within which is the vitellus, contain- 

 ing a germinal vesicle placed close to the surface. That part of 

 the vitellus in which the germinal vesicle is imbedded, the 

 germinal disc, is a small lens-shaped mass of clear protoplasm, 

 while the rest is mainly made up of food-yolk, through which 

 protoplasm is sparingly diffused. The yolk is of two kinds 

 yellow, composed of granular spheres, and white, formed of smaller 

 spheres enclosing highly refracting spherules. The yellow yolk 

 is more abundant, while the white yolk is arranged in a flask- 

 shaped mass running from the germinal disc to the centre of the 

 ovum, and several thin lamella concentric to this. 



The oviducts possess muscular walls, and are lined by glandular 

 and ciliated epithelium. 



The cloacal chambers of the two copulating individuals are 

 partially everted and the sperm passed into the oviduct. The 

 ova burst out of their capsules into the body-cavity, and are 

 taken up by the funnel of the oviduct. Before passing down 

 very far, they are fertilized by the fusion of a single sperm with 



8. Muscular System. The muscles may be classified as in 

 the frog (p. 208), but their arrangement is more complicated. 

 The presence of air-sacs largely increases the surface for attach- 

 ment of the muscles of flight, the most important of which take 

 origin in the sternum and its keel, and are inserted into the 

 humerus. The specific gravity of the body is diminished by the 

 presence of air-sacs, as also by the nature of the bones, and flight 

 is thus indirectly aided. 



Both unstriated and striated muscles are present, as in the 

 frog, and their structure and distribution are substantially the 

 same. 



9. The nervous system (Fig. 70) consists of cerebro-spinal 

 axis, cranio-spinal nerves, and sympathetic system. 



(1) The cerebro-spinal axis consists of brain and spinal cord, 

 enclosed in the neural canal and covered by membranes, but 

 closely fitting the cavities in which they are placed. 



The brain is large and rounded. As before (p. 210), it may 

 be divided into fore-, mid-, and hind-brains : (a) Fore-brain. 

 The thalamenceplidon contains a large slit-like third ventricle, 

 bounded by the lamina terminal-is in front, and the optic thalami 

 at the sides (united behind by a posterior commissure), while a 



