REGULATORY MECHANISMS 407 



nating against the cold or aestivating against the heat. So it 

 is found that the supreme and dominant animals in arctic 

 regions are the birds and mammals, while in the tropics, 

 reptiles can compete successfully with birds and mammals. 



It is interesting to notice that during most of the period 

 of incubation, the embryo chick is poikilothermous. It is 

 only shortly before hatching that it acquires the capacity 

 of maintaining a uniform temperature. The same is true of 

 new-born mice, which become homothermous by the tenth 

 day after birth. 



Another matter for which a regulatory mechanism has been 

 evolved in the vertebrates is the osmotic pressure of the blood. 

 Of aquatic invertebrates it may in general be said that their 

 body-fluids have roughly the same osmotic pressure and the 

 same percentage of salts as the water in which they live, and 

 that these vary as the water varies. It is interesting to find 

 that in the Selachii, the osmotic pressure of the blood is not 

 constant either, but varies with the water. The salts in the 

 blood are only about half as concentrated as in sea water, but 

 the blood of Selachians contains urea, which makes up the 

 difference. In the Teleosts, the osmotic pressure of the blood 

 is about one-third that of sea water, but it is kept more or less 

 constant. This regulation is more efficient in some forms than 

 in others ; the osmotic pressure varies with that of the sur- 

 rounding water slightly in the cod, varies more in the plaice, 

 and varies still more in the eel, which alternates between fresh 

 and sea-water. As a rule the osmotic pressure of the blood 

 of fresh- water Teleosts is lower than that of the marine forms. 



In the land- vertebrates, the osmotic pressure of the blood 

 is kept constant, and regulated by the kidneys, in spite of 

 variations in food and drink. 



In an animal like a Selachian, living in the sea, it is not of 

 much importance if water and salts be lost from the body, as 

 they can be replenished from the medium in which it lives. 

 In a land-vertebrate the case is different, and the loss of water 

 and salts is regulated. The importance of maintaining a 

 constant osmotic pressure of the blood lies in the fact that it 

 entails constancy in the concentration of salts, or in other words, 



