342 FPLEXNER. [VOL. XIV. 
The regeneration of longitudinally divided animals goes on 
with far less rapidity ; finally, however, the result is quite as 
perfect as in the other instances. What takes place at once 
after section is, as already stated, a curling of the halves, the 
unraveling of which may require two weeks or even longer. 
As the restoration of the removed parts precedes this unwind- 
ing the degree of regeneration may be inferred from the 
extent of the return to the normal form. 
The manner of the production of an anatomically complete 
individual is of much interest... Confining our description to 
the nervous system it is evident that the new segment could 
be produced in the growing half in one of two ways. Either a 
new and independent formation of the removed parts which 
become united with the remnant of the old system takes 
place, or the segment of the old system furnishes the starting- 
point of the regenerating half. 
The cellular changes following median division are similar 
to those which succeed to simple decapitation, in so far as the 
exposed segment of the nervous mechanism becomes sur- 
rounded with cells which are gradually differentiated into 
ganglion cells and fibers. This increase in cells which, under- 
going successive differentiation into neurones, gradually 
extends the nervous elements into the growing half, gives rise 
in succession to the cephalic ganglion, including commissure 
and nerve cord, thus projecting, as it were, the old system into 
the new part. There is, then, not a formation de novo of the 
removed segment, but a continuous outgrowth from the intact 
half (Fig. 9); the new cells do not arise by division of the old 
nerve (ganglion) cells, but come from other sources. 
The phenomena attending regeneration of the central 
nervous system have been studied in both vertebrates and in- 
vertebrates, and with somewhat varying results by different 
observers. Most investigators agree in having found that in 
vertebrates the capacity for reproduction of removed or injured 
parts is very slight or even 2/1 While Coén, von Kahlden, 
Sanarelli, and Friedmann found that defects in the cerebrum 
and cerebellum were replaced by connective tissue, and 
1 See Barfurth, Regeneration, Merkel-Bonnet’s Ergebnisse, i, 1891, 132. 
