REPRODUCTION AND SEX 461 



ductive even in larval life. Another set of contrasts is disclosed when 

 we compare {a) egg-lajang animals, whose eggs are fertilised quite 

 outside the body, as in salmon, to take one example out of myriads; 

 (6) egg-laying animals, whose eggs are fertilised at the moment of 

 liberation, as in frogs; (c) other egg-laying animals, whose eggs are 

 fertilised inside the body, as. in birds. Similarly, as to hatching, that 

 occurs outside the mother's body in the oviparous tj^s, e.g. birds 

 and most fishes. In other types the eggs are already hatching as they 

 are being laid, as in some lizards and snakes; and to this the awk- 

 ward term "ovo-viviparous" is applied. In yet other types the young 

 are liberated from the egg-envelope while they are still within the 

 body of the mother, as in the primitive Peripatus or in the common 



Fig. 60. 



Embryo of Peripatus in the Oviduct of the Mother. After Kennel. The 

 embryo (E) is bound to the wall of the oviduct (OV) by a linkage (PL) 

 which is distantly analogous to a mammalian placenta. The young may 

 remain for a year developing within the mother. 



Brown Lizard, two widely separated viviparous forms. Taking 

 mammals by themselves, we find that the Monotremes are actually 

 oviparous; the Marsupials are prematurely-bearing, with no true 

 placenta (save a beginning in Perameles). All the other mammals 

 (Placentals) have a more or less prolonged ante-natal partnership 

 between the mother and the unborn young. This is anticipated in 

 the ante-natal placenta-like connections in the oviducts of two 

 dogfishes and in several reptiles; but its perfected development 

 as a complex organ, effecting symbiosis between the mother and the 

 unborn offspring, is restricted to the Placental Mammals; and in 

 various orders of the placental mammals, this important organ has 

 been shown by anatomists to present various phases of evolution. 



Thus, broadly speaking, not only has the general development 

 of the mammals been in terms of mothering, but their main lines 



