INTRODUCTION. Xlll 



in the shapes of the crystals of different species, differences quite as marked 

 have been described in the crystals of the same species by different observ- 

 ers. Thus, the crystals from the blood of the guinea-pig have been recorded 

 by one as occurring occasionally as octahedra, by another as tetragonal, 

 by another as tetrahedra and rhombic plates and prisms, and by another 

 as six-sided plates; squirrels' crystals are almost invariably described as 

 hexagonal plates, but they have also been seen as needles and rhombic 

 prisms; crystals of the mouse are variously referred to as fine needles, or 

 six-sided plates, or small prisms; rats' crystals are described as tetrahedra, 

 prisms, rods, plates, needles, or hexagons. 



Even the processes for preparing protein crystals are, except those of 

 the hemoglobins, very limited in their range of usefulness, as has been 

 shown by the failure to obtain crystals of even corresponding proteins 

 save in a very limited number of instances. Moreover, the processes are 

 so far from perfect that crystalline condition per se is not a guarantee that 

 we have necessarily either a pure or a normal body; nor are such crystals 

 free albumin, free globulin, etc., but acetates or sulphates or other forms 

 of combination, etc. Moreover, when purification of proteins has been 

 sought by repeated recrystallization it is by no means clear that the proc- 

 esses have not given rise to abnormal substances through a stripping off of 

 very unstable or feebly combined radicals which constitute normal com- 

 ponents of the molecules, and which of course must contribute to giving 

 the substance its peculiar properties. In the case of the hemoglobins it 

 has been shown that decomposition products are formed at each step of 

 recrystallization. It has even been found that crystalline habit may be 

 so affected by recrystallization that a hemoglobin which normally appears 

 in the form of hexagons may crystallize only in rhombic needles and tetra- 

 hedra. 



We are not, however, without certain data which indicate differences 

 of corresponding proteins. The very fact of the breaking down of the 

 serum albumin and the serum globulin of the food during the processes of 

 digestion, when these substances are in a natural state, can not be due to 

 a non-absorbability, because in such form they may be rapidly absorbed 

 under appropriate conditions, and it is at least suggestive that the degra- 

 dation of the protein molecules must be, in part at least, for some important 

 purpose in relation to the synthesis of the proteins of the individual. To 

 what extent this disintegration is carried out we do not know, but from our 

 present knowledge it is probable that the molecules are broken down into 

 essentially the primary dissociation products. In what ways and from 

 what derivatives the proteins of the individual are built up is a matter of 

 speculation, but it seems at least that such analyses and syntheses mean 

 a differentiation of the proteins of the food and the corresponding proteins 

 of the animal. 



The best instance on record which positively indicates or shows chem- 

 ical differences in homologous proteins was brought to light by Kossel 

 and others several years (1904 et seq.) after this research was begun. The 

 differences in the protamines in elementary composition, in rotatory power, 



