DIFFERENTIATION AND PROTEIN SYNTHESIS 



SI 



two distinct sheets and is covered by small circular markings (Plate 4c). 

 Mitochondria are small and not particularly common in the cytoplasm 

 (Fig. 9). They posses a double-layered outer membrane enclosing an 

 inner chamber penetrated by what seem to be invaginations of the inner 

 membrane. According to Montagna (1956), mitochondria are often 

 difficult to see in the light microscope, but no problem arises in observing 

 them electron-microscopically. Numerous small vacuoles, often in 

 clusters, which may be identified with the Golgi apparatus (pp. 46-47) 

 are also visible in electron micrographs (Fig. 22e). 



Fig. 36. The cytology of basal layer cells. BM, basal membrane; 



N, nucleus; nu, nucleolus; m, mitochondrion; M, cell membrane; 



D, desmosome; P, dense RNP particles. 



The cytoplasm of the basal-layer cells is strongly basophilic and rich 

 in ribonucleic acid (Montagna, 1956 and Hardy, 1952). Electron- 

 microscopically the most striking feature is the presence of large numbers 

 of small (150-200 A diameter) dense particles (P in Plates 7 and 11). In 

 this respect these cells resemble many other kinds of rapidly-growing 

 cells: early embryonic cells in general, tumour cells, etc. Palade (1955) 

 has marshalled the evidence to show that the dense particles are in fact 

 the images of a ribonucleic acid-protein molecule (RNP). This matter is 

 discussed in detail on p. 108 et seq. 



The cells rest on and are attached to a basement membrane (BM in 

 Plates 7, 9 and 14B) the detailed structure of which is considered below. 

 Partly as a result of this attachment on one face and of their close-packed 

 condition, the germinal cells assume a columnar form in some situations. 

 The basement membrane itself is supported by a dense feltwork of collagen 

 fibrils. The follicles of growing hairs and feathers penetrate below the 

 general level of the base of the epidermis and become enclosed in a basket- 

 like network of circular and longitudinal fibrils, which, however, do not 



