544 
ZOOLOGY: R. G. HARRISON 
cases develop into a normal limb. Miss Byrnes^ some years ago called 
attention to this phenomenon in the case of the hind limb rudiment of 
Rana embryos, though the results of the present study, which has 
aimed to determine exactly the limits of the power to regenerate, show 
that it is much more restricted than she supposed to be the case. Per- 
haps, however, there is in Amhly stoma the same difference in regenera- 
tive capacity between fore and hind limb regions that Braus,^ found to 
be the case in the Anurans. Covering the wound with indifferent ecto- 
derm, or healing back the original ectoderm of the limb region, hinders 
this movement of peripheral cells toward the nodal point from which 
the new limb may arise, and definitely prevents development, provided 
that the extent of the wound includes the region of the anterior half 
of the sixth somite in addition to that of the third, fourth and fifth. 
In case the wound is but three segments in diameter, development 
may or may not be prevented but the proportion of negative cases 
is considerably larger than when the wound of the same size is left 
uncovered. 
Carried out upon a different form and on much younger embryos 
than the experiments of Braus, the present experiments afford additional 
evidence that the mesoderm cells of the limb region, while forming a 
definitely differentiated system as a whole, are nevertheless totipotent 
within that system as far as the skeletal and muscular elements of the 
limb are concerned. They can give rise to a perfect limb when placed 
in an unusual environment, even when the original arrangement of the 
cells is disturbed, as it necessarily is when the mesoderm alone is trans- 
planted; and a small part can develop into a whole. Other experi- 
ments, not described here, show, however, that at the time of operation 
differentiation has already begun to some extent, though it has evi- 
dently not become irreversible. The ectoderm merely serves as a cover- 
ing to the limb and no indication of any specific stimulus from this 
layer could be detected. Other structures in the region such as the 
myotomes, as shown by Byrnes (1898) and Lewis {op. cit.) or the pro- 
nephros, as found in the present investigation, do not affect the develop- 
ment or regeneration of the limb bud in any way. 
^D. Barfurth, Arch. EntwMech., Leipzig, 1 (1894). 
2 E. F. Byrnes, /. Morph., Boston, 14 (1898); Anat. Anz., Jena, 15 (1898); Arch. Entw- 
Mech., 18 (1904). 
3 P. Kammerer, Arch. EntwMech., 19 (1905). 
^H. Braus, Munchener Med. Wochenschr. (1905); Morph. Jahrh., Leipzig 35 (1906). 
5 W. H. Lewis, Anat. Record, 4 (1910). 
« H. Braus, Morph. Jahrh., 39, 194 (1908). 
^H. Braus, Ibid., p. 317. 
8E. F. Byrnes, Anat. Anz., 15 (1898). 
9 H. Braus, Morph. Jahrh., 35 (1906). 
