90 KERATIN AND KERATIN IZATI UN 



again into physical contiguity. Thus they are found wherever mesodermal 

 tissues contact ecto- or endodermal tissues (Fig. 12, p. 23). They are 

 apparent after various fixation procedures and are more dense after the 

 preparations are stained with such " electron-dense ' ' materials as phospho- 

 tungstic acid. Their origin and nature will be discussed further below. 

 Beneath the basal membrane are found bundles of collagenous fibrils, 

 the characteristic product of the dermis (Fig. 25). These fibrils are found 

 more or less regularly arranged in different sites and in different animals. 

 They may form a well-organized orthogonal pattern in amphibian larval 

 skin (Plate 9) which is smooth and free from structural disturbances 

 caused by hair follicles (Weiss and Ferris, 1954 and Porter, 1956) or less 

 well-organized bundles in mammalian skins (Ottoson et al., 1953; Selby, 

 1955; and Mercer, 1958). For the anatomy of the collagen deposits, see 

 Horstmann, 1959 and Salecker, 1944. 



The Development of Basal Membranes and their Role in the 

 Formation of Epithelia 



Since the epidermis (ectoderm) must early take over the task of holding 

 the embryo together, it seems clear that the ectodermal cells develop 

 self-adhesion at an early stage in embryonic development. Thus the 

 study of the establishment of the early epidermis offers a means to 

 investigate these fundamental events. The early work of Holfreter 

 (1947) and the electron microscopy of Weiss and Ferris (1954) and Porter 

 (1956) showed that the larval amphibian skin was useful material for this 

 purpose. These electron-microscope studies have mostly concerned the 

 organization of the dermal collagen fibrils. The present writer has examined 

 earlier stages, commencing before the appearance of the membrane, in the 

 tails of tadpoles of Xenopus and Rana. In a Xenopus tadpole (Stage 19, 

 Nieuwkoop and Faber, 1956) and similarly-developed Rana tadpoles the 

 extreme tip of the tail consists only of a single layer of ectodermal cells 

 enclosing a cavity containing very few cells of mesodermal origin. 



The cells are already showing the epithelia habit but, significantly they 

 are only closely adherent at their outside edges, the inner faces being widely 

 separated and highly convoluted (Plate 8). There is no basal membrane 

 and no dermal collagen. Later stages (developmental-wise) are found 

 nearer the head. Here the tail skin has acquired a defined epidermal 

 appearance, a basal membrane exists and the deposition of collagen has 

 commenced (Fig. 38) (p. 87). In these more advanced areas the dense 

 basal membrane (BM) everywhere follows the smooth undersurfaces 

 of the epidermal cells (Figs. 38 and 39) at a rather fixed distance. The 

 lighter layer (C) between it and the cell membrane (M) is continuous with 

 the lighter layer extending between the cells. It is probably missing on 

 the free surfaces facing the external environment where significantly may 



