180 



SCIENCE. 



[N. S. Vol. XXII. No. 554. 



SPECIAL ARTICLES. 



A FOOTNOTE TO THE ANCESTRAL HISTORY OF THE 

 VERTEBRATE BRAIN. 



The existence of a pair of newly recognized 

 ganglionated nerves, attached to the fore-brain 

 of that ancient group of fishes — the Selachians 

 —is, perhaps, of sufficient general interest to 

 justify a brief mention in Science. 



In recent numbers of the Anatomischer 

 Anzeiger (Vol. XXVI., Xos. 2/3 and 4/5) the 

 writer has described and illustrated the mor- 

 phology of such nerves in twenty genera, and 

 twenty-seven species, of selachians, and their 

 embryonic history in one. Similar nerves 

 have also been pointed out by AUis, Pinkus 

 and Sewertzoff in two of the Dipnoi (Pro- 

 topterus and Geratodus), and one ganoid 

 (Amia). So far as observations go these 

 nerves are absent in all other vertebrates. 

 The facts now made known, although in one 

 sense merely anatomical details, may, never- 

 theless, be looked on as constituting a footnote 

 to the great chapter in morphological science 

 dealing with the structure and development 

 of the vertebrate brain. 



The fore-brain is extremely modified in 

 structure, and so little understood that it is 

 desirable to gain any new facts bearing on its 

 organization. So far, comparative anatomy 

 and embryology combined have supplied only 

 fragmentary views as to its nature, the num- 

 ber of segments that may enter into it (or, 

 indeed, whether it is segmented at all), and 

 as to its line of development. Although the 

 solution of these matters is likely to take many 

 more generations, yet, in the meantime, de- 

 tails of structure hitherto unknown may help, 

 be it ever so little, in the anatomical analysis 

 of that region of the brain. 



The new nerves referred to above exist in 

 adult stages of selachians. They are, on the 

 one hand, connected with the fore-brain and, 

 on the other, with the olfactory epithelium. 

 They arise earlier in embryonic history than 

 the olfactory nerves, appearing on the an- 

 terior summit of the fore-brain near the 

 neuropore. Peripherally they intermingle 

 with the olfactory fibers, but, at all stages of 

 their existence, they are entirely independent 

 of those fibers, and they never come into rela- 



tion with glomeruli as the olfactory fibers 

 always do. 



As to their surface connections with the 

 brain wall, they exhibit two types : those hav- 

 ing a dorsal or semidorsal attachment to the 

 brain-wall, and those having a ventral attach- 

 ment. It is, however, to be borne in mind 

 that the superficial attachment of a nerve is 

 of small significance in determining its posi- 

 tion, as in the case of the fourth nerve, which 



Fig. 1. Brain of Squatina angeliis, from above, 

 natural size. 



emerges from the dorsal surface of the mid- 

 brain, while its neuroblasts lie in the ventral 

 zone. 



As an illustration of the first type, reference 

 may be made to Pigs. 1 and 2 showing, re- 

 spectively, the brain of the angel-fish (Squa- 

 tina angelus) and that of the spiny dog-fish 

 (Squalus acanthias), one of our commonest 

 selachians. 



In Pig. 1 the new nerve (n. nov.) is seen 

 attached by two roots to the dorsal surface 

 of the prosencephalon, and, bearing just in 

 front of the brain, a filiform ganglion (gn.). 

 Prom this point it runs like a delicate thread 

 along the olfactory tract, and dips into a 

 fissure separating the two great divisions of 



