REGENERATION IN VERTEBRATES 301 



Other cells of the wound and blastema (see also Schaffer, 1927; An- 

 dreasen, 1952). It is a means whereby developmentally superfluous 

 cells, and matrix as well, are rendered, completely or partly, and the 

 remains are carried to the outside. Large numbers of cells in various 

 conditions may be disposed of by this route. 



Such a route of disposal may explain the fate of resorbing stumps 

 and regenerates after denervation or irradiation with X-rays. In these 

 circumstances the regenerate and even the entire stump may be pro- 

 gressively destroyed and resorbed in a distal proximal direction (But- 

 ler, 1933; Schotte and Butler, 1941; Butler and Schotte, 1941). The 

 route of disposal could not be ascertained in these cases. Brunst 

 ( 1959 ) believes that giant cells which appear in great numbers after 

 irradiation are largely responsible for the reduction. According to our 

 observations, it is likely that the epithelium plays a major role. Previ- 

 ous results from our laboratory on resorbing denervated barbels of the 

 fish have already suggested such a route (Kamrin and Singer, 1955). 

 Dying cells may be attracted and move into the epithelium, as occurs 

 for phagocytes, or they may contact the epithelium and be drawn into 

 it through some special quality of the epithelium, such as "selectivity 

 affinity" or reactivity of epidermal cells '(Weiss, 1958). In the normal 

 development of a part, and in other circumstances such as those above, 

 there may literally be a stream of phagocytes, dead cells, and dying 

 cells entering the intercellular spaces of the epidermis (Figures 1, 5, 

 and 6). The products of their destruction may be utilized by the epi- 

 thelial cells or carried to the surface and excreted. 



In addition to disposing of cellular matter, the epithelium gathers 

 and may render scattered granules and materials of all sorts. Particu- 

 late matter and molecularly-dispersed substances presumably are car- 

 ried outward by a steady stream of fluid which may well up through 

 the epithelium from below. The wound area and the early regenerate 

 are highly edematous, and the fluid pressure may be great enough to 

 promote this movement. Electron-microscopic studies show that there 

 is no morphological barrier to such a stream. The adepidermal mem- 

 brane is incomplete, and enlarged spaces exist between adjacent 

 epithelial cells. Therefore, the fluid may ooze into the "pores" ( Figure 

 2) between cells of the basal epidermal layer and thence outward 

 through intercellular channels to the surface. As the fluid seeps among 

 the blastemal cells and percolates thence into the epidermis, it would 

 carry along substances of various size. It may be that delicate move- 

 ments of the epidermal cells assist the movement of the particles into 

 the epithelium. The larger particles may be seized by the epidermal 

 cells upon contact and the smaller move freely deep into the epithelial 

 spaces, where they may be acted upon by chemical emanations of the 



