THE ORIGIN OF VERTEBRATE!) ANIMALS 185 



strong sharp-pointed spines like a hedgehog. It 

 has an elongated tapering snout, a long tongue used 

 for licking up ants, and no teeth. The feet are 

 provided with long and strong claws. Echidna 

 burrows very rapidly, and is mainly nocturnal in 

 habit. It can endure long fasts, and even exist for 

 a month or more without food. 



Embryologlcal Evidence. — Other Mammals bring 

 forth their young alive ; but they are really developed 

 in the same manner as other Vertebrates, such as 

 the frog or bird, from a single cell or ovum ; and 

 the embryo is retained within the body instead of 

 being laid. 



The eggs of mammals are very small, that of the 

 rabbit being o. 116 mm. (yto inch) in diameter. The 

 curious point is, that the eggs of mammals develop 

 after the fashion of large eggs, and not in the 

 manner of small eggs. The difference in size of 

 eggs depends mainly on the amount of food yolk 

 contained in them ; a small egg, such as that of the 

 frog, develops entirely and directly into the embryo ; 

 a large egg, that of the chick for instance, is hin- 

 dered by the presence of food yolk, and becomes 

 constricted into two parts, the embryo and the yolk 

 sac. The egg of the rabbit develops as though it 

 had a large amount of food yolk, and forms a yolk- 

 sac. This fact is only intelligible on the view that 

 mammals are descended from ancestors having large- 

 yolked eggs. (Fig. 34.) 



Monotremata are of special interest in this respect. 

 It was long believed that their eggs were laid as by 

 birds, and in 1829, Professor Grant described, on 



