GENERAL CHARACTERS OF LIVING ORGANISMS 33 



demonstrated; i.e., each compound has a definite and 

 characteristic crystalline form, which is similar for 

 compounds of similar chemical configuration (law of 

 isomorphism). In colloidal compounds like proteins, 

 crystals are less easily produced, but under appropriate 

 conditions many of these compounds can be crystallized, 

 and it is then found that corresponding or homologous 

 proteins from different species form crystalline aggregates 

 which differ characteristically in their specific form- 

 characters. Specificity of crystalline form has been 

 demonstrated most clearly in the case of the haemoglo- 

 bins; i.e., the haemoglobin crystals of the domestic cat 

 dift'er in a definite and constant manner from those of 

 other species of the same family, and in different verte- 

 brates a general correlation between similarity of crystal 

 form and nearness of relationship can be recognized,^ 

 Such facts indicate that as the molecules unite in the 

 process of crystallization to form larger aggregates, 

 structures are built up having definite morphological 

 characters which are determined by the special configura- 

 tion of the haemoglobin molecule. The growing crystal 

 mass takes on definite form characters, like the growing 

 germ. We may assume that in the living cell, as it grows 

 and differentiates, similar conditions determine the 

 physical state assumed by those proteins which are laid 

 down as microscopic aggregates or deposits to form the 



^ Cf. Reichert and Brown, "The Crystallography of Haemoglobms," 

 Carnegie Institution Publication No. 116, Washington (1909); also 

 Reichert's paper, "The Germ Plasm as a Stereochemic System," Science, 

 XL (1914), 649. NuttaU's work with precipitin reactions demonstrates 

 a similar correlation between the chemical specificity of proteins and 

 blood relationship. (Nuttall, Blood Immunity and Blood Relationships, 

 Cambridge University Press^[i904].) 



