304 



PRINCIPLES OF EMBRYOLOGY 



character. The mass of cells which gather at the position of the wound, 

 and which eventually develop into the regenerated part, is known as the 

 'regeneration blastema'. 



It is probable that in morphallaxis the first process plays an important 

 role. In general, however, it is by no means the most important of the 

 three. Even in animals such as coelenterates, flatworms and ohgochaetes 

 in which morphallaxis occurs to a considerable extent, it has been shown 

 that the body contains a supply of undifferentiated cells (known as reserve 

 cells or neoblasts) which play a large part in the regeneration (Fig. 14.1). 

 The stimulus of the wound activates these cells. In the coelenterates those 



Figure 14. i 

 Longitudinal section through the gastric region o£ Hydra, with endoderm^ 

 on left and ectoderm on right and the mesoglea {Mes) between. A group ot 

 neoblasts or 'interstitial cells' is shown at C.i. At C.ep.i similar cells are 

 developing into epidermic (=ectodermal) cells, at C.n.i mto cmdoblasts, 

 and at C.b. into endoderm. (From Stephaii-Dubois 1951-) 



in the neighbourhood of the wound begin to divide and to produce the 

 tissue out of which the regenerate is formed, while in flatworms the 

 neoblasts even from further away migrate to the wound surface and give 

 rise to the blastema (Stephan-Dubois 1951). In higher animals such as 

 vertebrates, the direct outgrowth of the already existing tissues is only a 

 minor factor in regeneration. However, the nerve supply of a regenerate 

 is probably always produced in this way, by the sprouting and out- 

 growth of the nerve fibres in the stump. 



There has been a great deal of discussion about the origin and potenti- 

 alities of regeneration cells in vertebrates, and there is still by no means 



