52 NATURAL HISTORY OF VERTEBRATES. 



In many forms of reptiles, and in birds, the oviducts are not symmetrically devel- 

 oped, one only becoming functionally active ; but in mammals both oviducts are 

 developed, and generally fuse in their lower parts into a median chamber — the 

 uterus, in which the young are nourished by a placenta for a longer or shorter time. 

 Much difference prevails as to the mode of opening of the urogenital ducts, and their 

 relationship to the intestine. While they open independently of the intestine, as a 

 rule, in fishes, such is not the case in Amphibia, reptiles, and birds, where all open into 

 a common cloaca, which in its turn has a single aperture to the outside. In most 

 mammals, again, the urogenital ducts open in front of the anus into a urogenital 

 sinus, in connection with which are developed the external reproductive organs. 



In place of opening directly to the outside, the urinary ducts may first expand into 

 a bladder, serving as a reservoir for the urine, or there may be an ' allantoic ' bladder, 

 developed in a different way, although discharging the same function, with which the 

 urinary ducts have no direct connection except in the higher mammals. 



The mention of this allantoic bladder renders it desirable to explain a feature of 

 the development of the higher vertebrates, which was not alluded to at the beginning 

 of this sketch. In all reptiles, birds, and mammals, a protecting sac is early formed 

 around the embryo, known as the amniotic sac, and consequently these forms have 

 been designated Amniota, in contradistinction to the Anamnia (that is, the fishes and 

 Amphibia), which are destitute of it. The allantois is another fcetal membrane, which 

 is, however, developed from the intestine, and, being richly supplied with vessels, serves 

 for the respiration of the embryo, either by approaching the surface of the egg in 

 oviparous forms, or by coming intimately into contact with the blood vessels of the 

 uterus, as in the placental mammals. While the amnion is entirely discarded at birth, 

 this is only true of a part of the allantois, the remainder of which is chiefly converted 

 into the urinary bladder. 



In the foregoing sketch it has been thought desii-able to devote most space and 

 illustration to the discussion of those organs which are most easily studied by the 

 beginner in comparative anatomy, while enough has been said with regard to the other 

 systems of organs to enable the reader to put in their proper place isolated structural 

 details which he may meet with in the following pages. 



R. Ramsay Weight. 



