OSMOTIC PRESSURE IN ANIMALS AND PLANTS 567 



vestigated very fully, but for each species an approximately 

 constant value is found. Some fresh-water forms have pres- 

 sures of about 5, whereas marine species may go as high as 7 

 atmospheres. It may or may not be a coincidence that the 

 blood of the fresh-water reptilia is approximately isotonic with 

 the germ cells in the eggs of birds ; the latter group has of course 

 developed from the reptilia. 



(d) Amphibia. — The frog has afforded material for so many 

 physiological researches that the osmotic pressure of its blood 

 was determined among the earliest of such measurements . The 

 blood freezes somewhere about —0*46°, so the osmotic pressure 

 is about 5*5 atmospheres. Within the last few years, however, 

 an interesting research has appeared from the Upsala labora- 

 tories in which the gradual increase in osmotic pressure of the 

 frog, from spawn through tadpole to the adult condition, has 

 been traced. Thus the stages of the development of the race, 

 so far as osmotic pressure is concerned, are passed through in 

 each individual. In the amphibia, as in all classes from the 

 teleosts upwards, the osmotic pressure is delicately regulated 

 by the nephric system. 



(e) Birds. — Among his numerous researches in this domain 

 of physiology Hamburger determined the freezing-points of 

 the blood of the domestic fowl. Cryoscopic measurements of its, 

 of the duck's, turkey's, and rhea's were also made by the author. 

 It was found that the osmotic pressure of the blood of birds is 

 quite constant for each species, and is higher than that of 

 reptiles and amphibia, being about 7-2 atmospheres. The 

 osmotic pressure of the eggs of several kinds of bird was also 

 determined, and found to be remarkably constant also, provided 

 the eggswerefresh ; the magnitude was from 5 to 5* 5 atmospheres. 

 During incubation it was found that the pressure of the mixed 

 contents of the egg — and presumably of the embryo also — rose 

 progressively, owing no doubt to the increase in crystalloids 

 arising from the splitting up of complex colloidal molecules. 

 One is tempted to correlate the low pressures found in the adult 

 fresh-water reptilia with the pressures of closely similar magni- 

 tude found in the embryo bird. The latter group, abandoning 

 more or less watery surroundings and taking to the air, had to 

 economise in weight in many ways. Perhaps one of these 

 economies was effected by carrying their blood and tissue solutes 

 in more concentrated solution. 



