402 



NATURE 



iFeb. 27, 1879 



The present paper contains the resuhs of further investigations 

 on this point ; it deals also with some features in the develop, 

 ment of the vertebrate olfactory organ, and with certain questions 

 of a more general nature affected by the conclusions arrived at. 



The Development of the Olfactory Nerve 



The olfactory nerve of an adult vertebrate is usually described 

 as consisting of three parts — a proximal tractus olfactorius, an 

 intermediate bulbus olfactorius, and a distal nervus olfactotius, 

 connecting the bulb with the olfactory organ. Of these parts 

 the two former are commonly said to arise as a hollow diverticu- 

 lum of the cerebral hemisphere — the so-called olfactory vesicle 

 or olfactory lobe. The third part, the nervus olfactorius, is 

 described as arising at a later stage either from the olfactory 

 lobe, from the olfactory organ, or from the intervening meso- 

 blast. In consequence of these peculiarities in its mode of de- 

 velopment, the olfactory nerve is said not to bear the slightest 

 resemblance to the other cranial nerve, and to be in no way com- 

 parable with them. Dr. Marshall, however, finds, from an 

 examination of a large number of vertebrate embryos — chick, 

 dogfish, salmon, trout, axolotl, frog, and lizard — that Xht nervus 

 olfactorius is the first part to be developed ; that it arises at the 

 same time as the other cranial nerves and in the same manner ; 

 that it appears before the cerebral hemispheres, and consequently 

 arises from the original fore-brain. He finds further that there 

 is no trace whatever of an olfactory vesicle in the chick till the 

 end of the seventh day, or in the dogfish till stage O of Balfour's 

 nomenclature ; in the salmon and trout there is no trace of an 

 olfactory vesicle up to the time of hatching, nor indeed, for 

 some time afterwards. Mr. Marshall maintains that the olfac- 

 tory vesicle must therefore be regarded as a structure of merely 

 secondary importance ; and that the olfactory nerves, since in 

 their early stage they do not differ embryologically in any respect 

 from the segmental cranial nerves, must be regarded as the first 

 or most anterior pair of true segmental nerves. 



The Development of the Olfactory Organ 



This will, in the absence of figure, be treated very briefly ; 

 those points only being noticed which are of special interest in 

 connection with the conclusions arrived at in the preceding part 

 of the paper. 



The olfactory pits appear at almost the same time as the 

 visceral clefts ; or, to speak more accurately, they first become 

 conspicuous objects at, or very shortly after, the time when the 

 anterior visceral clefts become open to the exterior. This occurs 

 about stage K in the dogfish, and about the fiftieth hour in the 

 chick. 



In their early stages the olfactory pits present a striking 

 resemblance to the visceral clefts in position, shape, size, and 

 general relations ; their external apertures elongate and become 

 slit- like, and the direction of the slit, like that of the visceral 

 clefts, is at right angles to the longitudinal axis of the head. 

 These facts are best illustrated by the study of whole embryos, 

 and of longitudinal vertical sections.^ They come out with great 

 clearness in all the types of vertebrates examined, but with 

 especial distinctness in the axolotl and salmon. 



The development of the Schneiderian folds presents several 

 points of great interest, which can be most favourably studied in 

 the elasmobranchs. Attention has already been directed by 

 Balfour^ to the very early appearance of these folds. The 

 important point, so far as the present question is concerned, is 

 that these Schneiderian folds appear at the same time as, or very 

 shortly after, the first rudiments of the gills. In addition to this 

 identity in time, there is also identity in structure ; in both cases 

 development consists in the formation of a series of equal, 

 closely apposed folds, mainly epithelial, but involving the under- 

 lying mesoblast to a certain extent. These folds are in the two 

 cases —gills and Schneiderian folds — of the same width, the same 

 distance apart, have epithelium of the same thickness and same 

 histological character, involve the mesoblast to exactly the same 

 extent, and in exactly the same manner ; in a word, are structur- 

 ally identical. 



In the later stages the Schneiderian folds, like the gills, 

 receive a very abundant supply of blood-vessels ; and the relations 

 of these vessels to the folds, which are very peculiar and cha- 

 racteristic, are identical in the two cases. Even in the adult 



I For figures of whole embryos illustrating the points referred, vide 

 Parker, " On the Structure and Development of the Skull in Sharks and 

 Skates," Trans. Zool. Soc, vol. x. part iv., 1878; PI. 25, Fig. i; PI. 39, 

 Figs. I and 2 ; PI. 40, Fig. i ; and Balfour, o^. cit., PI. 7, Stage L. 



^ 0/>. cit., p. 184, and PI. 44, Fig. 14. 



elasmobranch there is a remarkable histological resemblance 

 between the gills and the nose. 



The facts above recorded concerning the development of the 

 olfactory nerve and olfactory organ point towards the same con- 

 clusions as to the morphology of these structures, viz., that the 

 olfactory organ is a visceral cleft ; that the olfactory nerve is the 

 segmental nerve supplying that cleft in a manner precisely similar 

 to that in which the hinder clefts are supplied by their respective 

 nerves ; and that the Schneiderian folds are gills.^ 



These conclusions, if accepted, vnll considerably simplify our 

 conception of the segmentation of the vertebrate head. As 

 there are no nerves or clefts in front of the olfactory segment, 

 the olfactory nerve must be taken as the most anterior nerve, and 

 the nose as the most anterior cleft. The next cleft is that in 

 front of the maxillo-palatine arch, of which a part probably per- 

 sists in the adult as the lachrymal duct : the segmental nerve 

 corresponding to this cleft is the third, or oculomotor nerve. 

 Next comes the mouth cleft, supplied by the ffth, or trigeminal 

 nerve ; and then in succession the clefts supplied by the facial, 

 glossopharyngeal, and pneumogastric nerves. This view of the 

 constitution of the vertebrate head is found to accord well with 

 the later researches of Prof. Parker on the morphology of the 

 skeletal elements of the head. 



Some at least of the labial cartilages will probably prove, on 

 this view, to be homologues of the extrabranchials, a comparison 

 that has already been made by Prof. Parker.* 



If the olfactory organs are visceral clefts, they must originally 

 have communicated with the mouth cavity. Indications of a 

 former connection of this kind are by no means wanting ; thus 

 in salmon embryos the alimentary canal extends forwards, so as 

 to underlie the nasal sacs ; as development proceeds, this ante- 

 rior prolongation of the mouth cavity gradually shrinks ; it 

 persists for a short time as a pair of csecal diverticula, which 

 ultimately disappear altogether. 



In conclusion, it may be noted that the Schneiderian folds 

 afford an instance, on the theory here maintained, of structures 

 originally hypoblastic in nature becoming, from changed circum- 

 stances, epibiastic. 



"On an Extension of the Phenomena discovered by Dr. Kerr 

 and described by him under the title of ' A New Relation between 

 Electricity and Light.'" By J. E. H. Gordon, B.A., Assistant 

 Secretary of the British Association. Communicated by Prof. 

 Tyndall, F.R.S. 



In November, 1875, Dr. Kerr announced in the Philosophical 

 Magazine, that he had discovered a new relation between elec- 

 tricity and light. He showed that when gla-s is subjected to an 

 intense electrostatic stress that a strain is produced which causes 

 the glass to act like a crystal upon polarised light. 



On Wednesday, February 5, 1879, I was working at this 

 experiment in the Royal Institution, and endeavouring, by means 

 of the electric light, to project the effect on a screen, in prepara- 

 tion for a lecture on the next day. 



In the experiment as described by Dr. Kerr, and which was 

 shown plainly on the screen, on February 6, the light is ex- 

 tinguished by the Nicols, and reappears when the coil is set 

 going. 



In the projection experiment a patch of moderately bright 

 white light, about 3 inches diameter, appeared on the screen 

 when the coil was worked. The images of the points inside the 

 glass were about l§ inches apart. On Wednesday, however, the 

 electrostatic stress was accidentally allowed to become strong 

 enough to perforate the glass. Immediately before perforation 

 there occiurred the effects which are the subject of the present 

 communication. 



First appeared a patch of orange-brown light about six or 

 seven inches diameter. This at once resolved itself into a series 

 of four or five irregular concentric rings dark and orange-brown, 

 the outer one being perhaps fourteen inches diameter. In about 

 two seconds more these vanished and were succeeded by a huge 

 black cross about three feet across, seen on a faintly luminous 

 ground. The arms of the cross were along the planes of polarisa- 

 tion, and therefore (the experiment being arranged according to 

 Dr. Kerr's directions) were at 45° to the line of stress. 



The glass then gave way, and all the phenomena disappeared 

 except the extreme ends of the cross, and the discharge through 

 the hole, where the glass had been perforated, was alone seen. 



The phenomena were seen by Mr. Cottrell, by Mr. Valter 

 /the second assistant), and by myself. A fresh glass plate was 

 • Cf. Dohm, " Ursprung der Wirbelthiere," p. 23. 

 = Proc.Z:>o\. Soc, vjI. x. part iv., 1878, p. 212. 



