No.1.] ZHE DEVELOPMENT OF BALANOGLOSSUS. 73 
the nervous system of Balanoglossus has been derived. In 
this respect Cephalodiscus is a more primitive form, and I 
strongly dissent from Lang’s view vzz., that the apparent 
primitiveness of Cephalodiscus is due to its sedentary life. 
That it has been modified to a great degree to fit it for its 
present surroundings seems to be true, but over and beyond 
this it seems to have diverged from the main stem of the 
Enteropneusta a little earlier than Balanoglossus. 
In the preceding portion of the paper I have called attention 
to the method of development of the central nervous system. 
The mass of cells lying in the median dorsal line of the collar 
drops out and is covered over by the ectoderm from the sides. 
Subsequently this mass of cells ts elongated to form the dorsal 
tnvaginated nerve cord of the adult. 
Now, if we looked upon the mass of invaginated cells as a 
cerebral ganglion, and interpreted the subsequent process as an 
elongation of this cerebral ganglion to form the nerve cord, 
we would be on the road to a comparison between the dorsal 
nerve cord of the Amphioxus and the cephalic ganglia of other 
forms. 
Gegenbaur in his text-book advanced such a view without 
reference, of course, to the forms under consideration, but the 
view has met with little favor. Morphologists have shown a 
preference for inverted Annelids or Nemerteans with lateral 
nerve cords. 
I have no serious intention of reviving Gegenbaur’s specula- 
tion, and have referred to the question in order to introduce it, 
and because at first sight the behavior of the dorsal cord of 
Balanoglossus seemed to lend itself to such a view. Two very 
important facts seem to refute such an 7¢xterpretation of the 
facts in the case we are considering. 
I believe it to be a great error to speak of the zxvaginated 
dorsal nerve cord of Balanoglossus as equivalent to the whole 
of the dorsal cord of the higher chordata ; because : 
The invaginated nerve cord of Balanoglossus stretches through 
only a single metamere of the body. 
The invaginated nerve cord does not give off or receive lateral 
nerve fibres along its length. 
