DIFFERENTIATION AND PROTEIN SYNTHESIS 



S7 



Tail tip 



Ectoderm 



Stage 1 . Ectodermal cells showing 

 very convoluted surfaces indicative 

 of poor and impermanent adhesion. 

 No adhesion to or between meso- 

 dermal cells. 



Stage 2. Formation of close contact 

 between those portions of the cell 

 membranes near to the outer surface. 

 Further in from the surface wide 

 gaps and convoluted surfaces persist. 

 Traces of intracellular fibrils. 



Stage 3. Further differentiation of 

 surface cells. Appearance of (1) 

 mucin cells, (2) ciliated cells and 

 (3) definite deposits of intracellular 

 fibrils. 



Stage 4. Epidermis increases in 

 thickness and basement membrane 

 appears. Cells in basal layer contain 

 both filaments and endoplasmic 

 reticulum. 



Stage 5. Appearance of first colla- 

 gen fibrils beneath basement mem- 

 brane. Mesodermal cells increase 

 amounts of organized reticulum. 



Stage 6. Rapid build-up of colla- 

 gen beneath basal membrane, in- 

 creased synthesis of intracellular 

 fibrils, fading of reticulum in surface 

 cells. Mesodermal cells recognizable 

 as fibroblasts F (much rough sur- 

 faced reticulum) and muscle cells 

 (little rough surfaced reticulum and 

 muscle fibres M). 



Fig. 38. Shows the succession of cell types noted on passing from the 

 extreme tip of a tadpole tail (Xenopns and Rana) towards the head. The suc- 

 cession also represents stages in differentation from a poorly-differentiated 

 condition at the tip to a well-defined state of differentiation further head- 

 wards and may thus be regarded as a time sequence also. Six stages are 

 recognizable and are indicated (l)-(6) on the l.h.s. See Plate 8. 



