94 INTRODUCTION TO EVOLUTION 



ligation of the problem by Fisher and Eakin (1957) has cast doubt on the 

 correctness of this interpretation. These investigators found that at no time 

 does the chick embryo actively excrete ammonia, though a low and fairly 

 constant concentration of it is present in both embryonic tissues and yolk. 

 They also found that the urea arises entirely from the amino acid arginine, 

 and that in urea excretion the liver and kidney do not function in the man- 

 ner characteristic of amphibians and mammals. Furthermore they found 

 that as soon as the kidney starts functioning (at about the fifth day), the 

 excretory product is uric acid as in adult birds. Evidently, then, the chick 

 embryo does not recapitulate ancestral modes of excretion. 



Turning to the invertebrates for a moment, we note that insects dif- 

 fer from most other invertebrates in the high proportion of nitrogenous 

 waste excreted as uric acid. Like terrestrial vertebrates, most insects are 

 faced with the problem of conserving the body's supply of water. 



The Internal Environment 



We have noted a variety of fundamental similarities (homologies) in 

 chemical structure and in metabolism. Perhaps no similarity is more strik- 

 ing than the similarity in composition of the blood plasmas of varied ani- 

 mals. The plasma is the fluid portion of blood. It contains proteins in 

 solution; the similarities and dissimilarities of some of these form the ba- 

 sis of the serological tests to be discussed in the next chapter. But aside 

 from its proteins, plasma is essentially a salt solution. Salts of sodium, 

 potassium, calcium, and magnesium are predominant. Of these salts our 

 friend of the dining table, sodium chloride, occurs in greatest abundance. 

 At this time we are particularly interested in the fact that the relative 

 proportions of these various salts are strikingly similar in the blood plasmas 

 of most animals, not only vertebrates but also most invertebrates, insofar 

 as the latter contain body fluids correctly described as blood. Here is an- 

 other of those fundamental similarities running through the animal 

 kingdom. 



An additional point of interest is the fact that the relative proportions of 

 the various salts in blood plasma resemble the proportions with which these 

 salts occur in sea water. This similarity may be of significance. There are 

 various reasons for believing that life began in the sea — that the first or- 

 ganisms developed in the environment formed by a dilute salt solution, 

 which is what sea water is. Even today marine protozoa live completely 

 immersed in such a salt solution, and in such simply constructed crea- 

 tures as sponges and jellyfishes (Porifera and Coelenterata) all tissues, 



