4 ON VERTEBRATE ANIMALS 



it, is conducted to the respiratory organs; arterial blood is mixed 

 with venous blood. 



The respiratory organs of vertebrate animals are gills or lungs. 

 Vertebrates alone inspire by the mouth, and such as have lungs 

 exspire by the mouth also. 



In all vertebrate animals kidneys are present, parts by which 

 the urine so rich in nitrogen is secreted, and of which the internal 

 structure consists of fine tubules. In the lower vertebrates these 

 tubules unite to form branches which open into excretory ducts 

 (ureteres) running along the whole kidney; in birds and mammals 

 they unite to form bundles pyramids, which are arranged around 

 the cup-like commencement of the ureters. The two ureters, again, 

 often terminate in a bladder in which the secreted fluid is collected 

 before being discharged. 



As to the sexual organs, we have already stated above, that the 

 sexes are constantly distinct. Not by any means, on that account, 

 however, is there a copulation in all, but sometimes the eggs, as in 

 most fishes, are impregnated after they have been laid. The ovary 

 is single or double (paired). In this organ the eggs are formed, 

 and leave it in order to undergo further development in another 

 situation. With most vertebrate animals this situation is external 

 to the body of the mother, and the development depends upon 

 external conditions of light, water and warmth: according to the 

 difference of these conditions the egg may require a longer or 

 shorter period for the development of the germ ; the germ alone of 

 a future independent being is present at the time of birth, or of the 

 separation of the egg from the body of the parent animal. In other 

 cases the eggs, so to speak, are brooded within the parent body 

 itself; and here the birth, which terminates the period of foetal life, 

 is contemporaneous with the relinquishment of the foetal enve- 

 lops. Such animals are usually named viviparous 1 , but the limit 

 between these and the oviparous cannot always be defined with 

 precision. Most fishes and reptiles and all birds are oviparous. 



In the yolk of eggs, that still lie in the ovary, there is seen a 

 small transparent vesicle or a cell, which is surrounded by a ring 

 or by a small accumulation of a granular mass. This germ-vesicle, 



1 BURDACH names these animals nudipara, Die Physiologic ah Erfakrungswissen- 

 schaft, ii. Leipzig, 1828, s. 45 (2te Auflage, 1837, s. 48). 



