NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 271 
true, is easier for occipital than for pre-occipital segments. The seriai 
homology of occipital with trunk segments is not generally questioned 
at present. A comparison of the integral parts of occipital and 
trunk metameres shows that the belief in their serial homology is well 
founded. It must, however, he admitted that occipital metameres 
show no evidence of either excretory or reproductive organs. Never- 
theless we may readily believe from the evidence of these organs in the 
gill region of Amphioxus that this is a coonogenetic loss in the Ver- 
tebrate series. The chief grounds for belief in the homology of trunk 
and occipital metameres are these: (1) Occipital somites with their 
(2) ventral nerves are undoubtedly the serial homologues of trunk 
somites with their ventral nerves. This evidence alone has convinced 
most morphologists. But there are still other reasons. With our 
present knowledge, we may, I think, affirm that (3) dorsal occipital (or 
cranial) and dorsal spinal nerves are serial homologues. One by one, 
since the discovery by Schneider (’79) of ventral nerves in Amphioxus, 
the differences between dorsal spinal and cranial nerves, which were 
at one time, or another maintained, have been with increased compara- 
tive embryological and anatomical knowledge shown to be unessential. 
The evidence given by Schneider (79), Hatschek (’92), and van Wijhe 
(93) shows that dorsal nerves, as seen in Amphioxus, are mixed in 
function, innervating the skin and splanchnic musculature, while ventral 
nerves are motor in function, innervating somatic musculature. The 
typical cranial nerves of Oraniota, viz. V, VII, IX, and X, are mor- 
phologically comparable with the dorsal nerves of Amphioxus, and are 
therefore to be regarded, as Balfour for other reasons regarded them, 
more primitive than the spinal nerves, which lack the lateral and dor- 
sal (except in Cyclostomes) cutaneous branches.: The recent researches 
of von Lenhossék (90), Ramon y Cajal, and Kölliker, by demonstrating 
the existence of non-ganglionic fibres in the dorsal spinal nerves of 
Jraniota, which by their relations must be regarded as motor in func- 
tion, have shown that in this respect spinal nerves do not differ from 
cranial. Moreover, in view of the evidence given by Goronowitsch (92), 
Sewertzoff ('95), Neal (97), and Miss Platt (?97), it san no longer be 
1 The place of these branches has been usurped by the lateral branches of the 
vagus, as I believe has been su -ested by Eisig. The advantage in greater cen- 
tralization is obvious. If it be true, and it is generally admitted, that cranial nerves 
receive cells from the skin while the spinal nerves do not, an explanation of 
this also is seen in the extension of the vagus and the concomitant loss to 
spinal nerves, 
