Analysis of Rate of Regeneration 417 
entiated and embryonic in character. There are three possible 
sources one or more of which may have contributed to the pro- 
duction of these undifferentiated cells. 
1 If totally undifferentiated cells are present in the tail, those 
in the immediate vicinity of the injury may have multiplied and 
formed a layer of cells over the surface of the wound. 
2 There are in the tadpole tail many cells of a partially undiffer- 
entiated character whose normal functions are to continue the 
growth and carry forward the repair of the special tissue to which 
they belong. The cells of the chorda epithelium of the noto- 
chord and the large nucleated cells in the connective tissue are 
examples of this type, but without doubt the cells of the basal 
layer of the ectoderm are the most numerous and most active 
in this category. Barfurth (’03) describes these ectoderm cells 
as dividing and spreading over the cut end of the tail. It seems 
quite probable that the first layer of cells which covers the wound 
is from the ectoderm; but it is also evident from the sections 
studied that the large nucleated cells of the connective tissues 
as well as those of the chorda epithelium produce undifferentiated 
cells of the kind destined to become the connective tissue and the 
core of the notochord, before the rate of regeneration has reached 
the end of its first low interval. 
3 There is yet another way in which the ‘undifferentiated cells 
of the regenerating tadpole tail comes into existence. It is 
described by Barfurth and others and applies in this connection 
to muscle tissue only. For present purposes it is sufficient to 
refer to it as a process, akin to budding, whereby undifferentiated 
cells called sarcoblasts and destined to become new muscle fibers, 
are produced from a nucleus and a little protoplasm of an old 
muscle fiber. 
It is evident that some time must be consumed in the produc- 
tion of this layer of undifferentiated cells. Later a relatively 
small number of cell divisions occurring throughout this layer 
of new tissue will cause masses of cells to be protruded beyond 
the healed surface of the wound. Such masses of cells are the 
beginnings of measurable regeneration. From the study of 
the histological changes during regeneration it is shown that 
