No. 2.] DOUBLE-HEADED FORMS. 339 
months of July and August. As this study was begun at Woods 
Holl in 1895 during the corresponding months and completed 
upon material forwarded from Woods Holl to Baltimore in the 
summer of 1896, this comparison was easily possible. 
The especial purpose of this inquiry was to ascertain the 
manner in which the nervous (cephalic) ganglia and nervous 
cord were regenerated under the several conditions mentioned, 
and also what the new relations were which arose in artificially 
produced monsters. A few words regarding the anatomy of 
the normal parts may properly precede a detailed account 
of our study. 
The nervous system of the planarian with which I have 
worked consists of a cephalic and a trunk portion (Fig. 1). 
The cephalic extremity consists of two bulbous enlargements, 
lying near the lateral edges of the animal, on a line with the 
eye-spots. These are connected by a commissure which is 
somewhat smaller in dimensions than the ganglia, although it 
does not differ in structure from these parts. Passing out- 
wards towards the caudal extremity are two cords, one pro- 
ceeding from each ganglion, which run ina parallel direction 
quite to the end of the animal. These cords are connected 
by semicircular commissural processes which come off at 
somewhat irregular intervals. From the cephalic extremity, 
as well as from the nervous cords, nerves pass outwards to the 
periphery of the body. 
There is no essential difference in the microscopical struc- 
ture of the ganglionic masses and the nervous cords. Each 
consists of a bundle of very fine non-medullated nerve fibers in 
close proximity to nerve cells. These cells — ganglionic cells 
—are more numerous in the cephalic ganglia. But cells 
similar in type and appearance exist in the cords, and may 
even be found, although much more sparsely, in the nerves 
themselves. In the ganglia they exist within the bundles of 
nerve fibers, grouped in the center, scattered in the periphery 
but most thickly placed in the outermost zone. In the cords 
they are found quite regularly disposed among the fibers. 
The process of regeneration was studied in all stages, begin- 
ning twelve hours after decapitation, until the complete restora- 
