CHAPTER XI 

 BROOD-CARE AMONG MAMMALS 



MAMMALS are not only the highest group of the animal kingdom, 

 but one of the latest products of evolution. At a time which is not 

 very remote in geological history, a set of reptiles slowly assumed 

 the mammalian characters, and there is no reason to doubt but that 

 these ancestral reptilian -mammals laid large eggs like living reptiles. 

 Some of the living reptiles, like some living fishes, retain the eggs 

 in the body until they are almost ready to hatch, and so secure 

 for them warmth and protection much more certainly than in 

 the most cunningly devised nest. Some reptiles even keep the 

 eggs in the body until they hatch. It is most easy to understand 

 what now happens in mammals if we suppose that their reptilian 

 ancestors had acquired this habit of egg retention. The lowest 

 living mammals, the duck-billed mole and the spiny anteater of 

 Australia, still lay rather large eggs, but retain them in the body until 

 they are nearly ready to hatch. The marsupials and all the higher 

 mammals not only retain the eggs in the body but change the way of 

 feeding the embryo in a fashion that is foreshadowed even in some 

 fishes. In ordinary large eggs which contain enough food stored up 

 as yolk to nourish the young until it is hatched, the blood-vessels 

 of the growing embryo spread out over the yolk just under the 

 eggshell and absorb oxygen from the air through the shell as well 

 as food from the yolk inside the shell. When such an egg lies in 

 contact with the wall of the egg-duct of the mother, the supply 

 of oxygen for the embryo must be picked up from the blood of the 

 mother. This has led to two changes. In the first place, the embryo 

 picks up from the blood of the mother not only oxygen but the food 

 it requires, so that the yolk is no longer necessary ; and in the second 

 place the eggshell becomes thin, soft and membranous so that the 

 connection between the blood of the mother and of the embryo 

 becomes closer. Most of the marsupials have remained in this stage. 

 They have eggs that are smaller in proportion than those of reptiles 

 or than those of the lowest mammals, but much larger and containing 



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