344 THE RABBIT. 



These and other features in the development of Maim nals, which 

 will be described more fully later on in this chapter, are best 

 explained by supposing that existing Mammals are descended 

 from forms in which a greater amount of yolk was present, and 

 in which the eggs were consequently of larger size ; and that, 

 in accordance with the Recapitulation Theory, existing Mammals 

 have consequently an inherited tendency to develop after the 

 manner of forms with large eggs. The lowest group of 

 Mammals now living, the Monotremes of the Australian region, 

 afford strong evidence of the truth of this view, as, unlike other 

 Mammals, they are oviparous, laying eggs about the size of 

 olives, and closely resembling the eggs of many Reptiles. 



The young rabbit at birth has not yet completed its develop- 

 ment. Its eyes are closed, like a kitten's ; and it is quite 

 incapable of obtaining food for itself, being dependent for 

 a time on the supply of milk afforded it by the mammary 

 glands of the mother. Using the word gestation for the whole 

 period of development, from the first appearance of the embryo 

 to the time when the young animal becomes capable of indepen- 

 dent existence, two stages may be distinguished in it: — (i) 

 uterine or placental gestation, which comprises the period prior 

 to birth, during which the embryo is nourished by osmotic 

 interchanges between its blood and that of the mother; and 

 (ii) mammary gestation, which embraces the period after birth, 

 during which the young animal is nourished by the milk yielded 

 by the mother. 



The relative lengths of these two periods vary greatly in 

 different Mammals. In the Marsupials, a lowly organised group, 

 the uterine gestation is very short, the young animal, as in 

 the kangaroo or opossum, being born at an early stage, of small 

 size, and very imperfectly developed ; while in the higher Mam- 

 mals the period of uterine gestation is a much longer one, and 

 the young animals at birth are of larger size, and more com- 

 pletely formed. 



THE EGG. 

 1 . Formation of the Egg. 



To study the mode of formation of the ova in the rabbit it 

 is necessary, as in the chick and frog, to take, not adult animals 

 nor even new-born young, but embryos at an early stage of 

 development. 



