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organs become shut off from the surface by the development of the 
protecting canals. With regard to the development of these canals I 
have nothing to add to that which is already known. 
The backward growth of the lateral nerve along the whole length 
of the body is one of the most curious circumstances in Vertebrate Em- 
bryology. How comes it that a nerve which is a branch of a cranial 
nerve complex, innervates a region comprising the greater number of 
the segments of the body? 
In that portion of the head in front of the vagus the lateral line is 
innervated not by branches of one, but of four cranial nerves. That is, 
in the four or five segments in front of the region of the vagus nerve, 
there is primitively a »lateral nervec for every segment. Whereas in 
the body posterior to the origin of the vagus there is only one »lateral 
nervec which supplies the lateral sense organs of a very considerable 
number of segments. Is this a primitive state of things? 
For the elucidation of these problems it must be noticed that 
developmentally the sense organs of the lateral line are segmental, — 
in every segment of the body one pair of these sense organs is developed. 
No segment of the body, from the first segment of Van Wijhe back- 
wards, is an exception to this rule. Later more than one pair may be 
developed in some or all segments !, some may disappear, the arrange- 
ment also, may become complicated, but in all cases in the embryo 
the organs are segmental. Hence we may call these organs the seg- 
mental sense organs. 
Eisig in his valuable paper on the »Seitenorgane der Capitelliden« 
has drawn attention to the above point, originally discovered by Stan- 
nius, Malbranc and Solger, and to the further fact that in all 
essential points the side organs of the Capitellidae agree with those of 
Vertebrates. The one difference which obtains between the two sets of 
organs concerns the nerve supply. Eisig has shown that in the Capitel- 
lidae there are a pair of nerves in each segment to the corresponding 
segmental sense organs. This simple state of things does not exist in 
Vertebrates. But traces of such an arrangement are to be found. And 
indeed I hope to show that such a condition must be taken to have 
been the primitive one in Vertebrates. 
The segmentation of the vertebrate head, marked out by the course 
of the segmental nerves, is now universally accepted. In order to 
demonstrate the truth of the theory mentioned above it is necessary to 
know how many segmental nerves exist in the Vertebrate cranium. 
1 Since writing the above I have found that the number of sense organs is in- 
creased in the embryo by division of the primitive segmental ones. I think Malbranc 
has already recorded this in adult Amphibia. 
