306 UNITY AND DIVERSITY IN BIOCHEMISTRY 



"IV. Cells sometimes become transformed into bodies no longer 

 possessing all the characteristics of a cell. Cells (together with these 

 transformed cells, if present) are the living parts of organisms : that is, 

 the parts which accomplish the synthesis of new material. Cellular 

 organisms consist of nothing except cells, transformed cells, and 

 material extruded by cells and by transformed cells (except that in 

 some cases water, with its dissolved substances, is taken directly from 

 the environment into the coelom or other intercellular space). 

 "V. Cells are to some extent individuals, and there are therefore two 

 grades of individuality in most organisms ; that of the cells, and that of 

 the organism as a whole. 

 "VI. Each cell of a many-celled organism corresponds in certain respects 



to the whole body of a simple protist. 

 "VII. Many-celled plants and animals probably originated by the 



adherence of protist individuals after division." 

 The fact that cells are all of essentially the same nature is described in the 

 first part of this book under the heading of the biochemical unity of organ- 

 isms, and this idea implies also for the biochemist the similarity of each cell 

 of a multicellular organism to a monocellular organism. The idea that 

 organisms are made up of cells and transformed cells, for the biochemist, 

 corresponds to the concept of the biochemical diversity of the cells of the 

 same organism, all having however the same genotype. To illustrate the 

 great variety of biochemical differentiation in cells having the same genotype, 

 we shall take as our example an adult mammal such as man. The cells of the 

 ectoderm become differentiated into the cells of the epidermis, of the glands 

 of the skin, of the adenohypophysis, into nerve cells and modified nerve 

 cells such as those of the neurohypophysis or the suprarenal medulla, etc. 



To the biochemist, the epiderm cell in mammals is characterized par- 

 ticularly by the biosynthesis of keratin, a protein rich in S-S linkages. This 

 type of cell is also one of the sites of synthesis of cholesterol. The cells of 

 the sebaceous glands are notable in bringing about the rapid transformation 

 of the whole of their content into a mixture of many lipides, saturated 

 alcohols, and hydrocarbons, etc. 



Among the large polyhedral cells of the adenohypophysis we find several 

 types : cells whose cytoplasm is only slightly chromophilic, acidophilic 

 cells or eosinophils (a cells), and basophilic cells (^ cells). The eosinophils 

 are differentiated in that they synthesize two hormones, somatotrophic 

 (growth) hormone, and the lactogenic hormone (luteotrophin). The basophi- 

 lic cells biosynthesize thyrotrophic hormone, adrenocorticotrophic hormone 

 (ACTH), follicle stimulating hormone and the luteinizing hormone. It was 

 once believed that the hormones of the anterior hypophysis were all proteins, 

 but this has been shown to be false for ACTH and is very doubtful where 

 the thryotrophic hormone is concerned. 



