No. 2.] DOUBLE-HEADED FORMS. 343 
Schiefferdecker failed to observe regeneration in the spinal 
cord of mammals, Danilewski! claims to have demonstrated in 
a frog in which one cerebral hemisphere had been removed 
nine months before, a newly formed cerebral mass containing 
young nerve cells. 
To the proof furnished by Mingazzini? that in tunicates 
complete restitution of the brain, etc., takes place, may be 
added the observations of Friedlander on the manner of the 
reproduction of the central nervous organs in certain worms. 
According to this author, the restoration takes place by an 
outgrowth from the remaining normal portion. It is, however, 
not clear from Friedlander’s® study just what the origin of the 
nerve cells may have been, nor what is the nature and origin 
of the leucocytes which give rise to a compact, cellular new 
tissue. 
My studies of the manner of regeneration of the central 
nervous organs of the planarian are capable of a different inter- 
pretation. There is no evidence of an outgrowth proceeding 
from the intact and non-degenerated nerve cords, nor is there 
any indication of a proliferation of the nerve cells existing 
among their component fibrils. The new organ results from a 
new growth of cells which originate independently of the old 
nervous system and in close proximity to the epidermal cover- 
ing. They are not the fully formed and completely differenti- 
ated epidermal cells ; for in these there were never found active 
changes which could be regarded as regenerative in character. 
It seems much more probable —although complete proof can- 
not at present be adduced — that the actively dividing elements 
are of the nature of Arsazz cells, which, while destined to give 
rise to the surface epithelium, are still capable of being 
transformed into neuroblasts. 
A simple outgrowth from the several nervous cords can be 
excluded. For, besides the fibers which compose the ganglia, 
1 See Barfurth, Regeneration, Merkel-Bonnet’s Ergebnisse, iii, 1893, 171. 
2 Ueber die Regeneration bei den Tunicaten, Bol. della Societa di Naturaliste 
in Napoli, 1891, ser. 1, vol. v, p. 76. Ref. Merkel-Bonnet, Ergebnisse, i, 1891, 
122. 
3 Ueber die Regeneration herausgeschnittener Teile des Centralnervensystems 
von Regenwiirmern, Zeitschr. f. wissenscls Zoologie, 1x, 1895, 249. 
