418 WILSON. [Vot. III. 
separate. The latter, two sections further back, passes through 
a ganglionic region and shows the enlarged cords joined by a 
narrow transverse bridge of neural tissue. This alternate sepa- 
ration and junction of the neural cords can be traced for a con- 
siderable distance behind and in front of the sections figured. 
All my preparations indicate that the transverse bridge of tissue 
uniting the ganglia is a product of the neural cords and has no 
connection with the superficial ectoblast. 
I can add little to Kleinenberg’s account of the histological 
differentiation of the ventral cord. The fibrillar substance 
appears some time after concrescence of the neural cords, appear- 
ing in cross-sections of the ganglionic regions as a clear punc- 
tate, bilobed mass on the dorsal side just beneath the mesoblas- 
tic sheath, which is still very thin. On tracing the sections 
forwards the bilobed fibrillar mass grows steadily larger, and 
may be seen to be separated into two distinct bundles in the 
commissural regions, the interval between the two bundles being 
occupied by a triangular mass of cells that extends upward 
from the nerve-cells on the ventral side and comes into contact 
above with the dorsal mesoblastic investment. 
The neurochord first distinctly appears in the ganglionic 
regions in the upper part of the fibrillar mass, and in its earliest 
distinguishable condition already shows the three fibres charac- 
teristic of the adult. Its precise origin is difficult to determine. 
In the ganglionic region it unquestionably lies in the fibrillar 
mass, and may often be seen to be separated by a distinct line 
from the mesoblastic investment. In the commissural region it 
becomes lost in the apex of the mass of cells that separates the 
two halves of the fibrillar mass. All my preparations indicate 
that the giant-fibres arise by direct differentiation from the 
dorsal portion of the fibrillar mass, and not from the mesoblastic 
investment, —a conclusion which is entirely opposed to Vejdov- 
sky’s apparently clear and careful account (No. 44, p. 93). 
There can be little doubt, however, from recent anatomical 
studies (see Nos. 17, 37, 42) of the essential correctness of the 
original view of Leydig and Claparéde, according to which the 
colossal fibres of the neurochord are specially modified nerve- 
fibres, which have probably assumed a supporting function. 
