341 



Chaptek v. 



THE deyelop:mext of the eabbit. 



PRELIMINARY ACCOUXT. 



The rabbit may conveniently be taken as a typical member of 

 the Mammalia, the- bigbest group of Vertebrates. 



Mammals difiFer notably from the animals dealt with in the 

 previous chapter in being viviparous; that is, in bringing forth 

 their young alive, instead of laying eggs. This difference, though 

 at first sight a striking one, is really of but secondary importance ; 

 and the actual processes of development of the Mammal are 

 effected in the same way as in the oviparous Vertebrates. 



Amphioxus and the frog lay their eggs in water, and the eggs 

 are not even fertilised until they have left the body of the mother. 

 In the Bird the eggs are fertilised as they leave the ovary and 

 enter the oviduct, and the earliest stages of development are 

 effected during the jjassage of the egg down the oviduct ; the egg 

 at the time of laying having already been developing for eighteen 

 hours or more. However, although development, in the case of 

 the bird, commences while the egg is still within the parent, it 

 is only the earliest phases — segmentation of the egg and the 

 establishment of the germinal layers — that are effected in this 

 position ; the formation and development of the embryo itself not 

 commencing untU after the egg has been laid. 



In the Mammal, on the other hand, the ovum is retained 

 within the oviduct and uterus of the mother for a very much 

 longer time ; and the whole embryonic development is completed 

 before leaving the parent ; the young Mammal at the time of 

 birth being fully formed, and of considerable size, though not 

 yet fit for independent existence. 



The rabbit embryo, like that of other animals, is developed 

 from an ovum or egg, which, as in other animals, is a single 



