604 TRANSACTIONS OF SECTION D. 



of the origin of secondary sexual characters in this way, although such characters, 

 once in existence, certainly vary spontaneously. 



The peculiarities of secondary sexual characters, their confinement to one sex 

 and to the period of sexual maturity, and the influence of castration upon them, 

 are the results of a special nexus between them and the sexual organs. Experi- 

 ment indicates that this nexus is a chemical substance in the blood, produced by 

 the testes. This substance cannot have been the cause of the evolution of the 

 organs, and the existence of the nexus is more in accordance with the theory of 

 external stimulation as the cause than with any other theory. 



4. The Pineal Sense Organs and Associated Structures in Geotria and 

 Sphenodon. Bi/ Professor Arthur Dendy, D.Sc, F.L.S. 



In the New Zealand lamprey (^Geotria austvalis) the pineal sense organs have 

 very much the same structure and relationships as in the European genus 

 Petromyzon. The * pineal eye ' is unusually highly developed, and there can be 

 but little doubt that it is a functional sense organ. It lies upon the roof of the 

 thalamencephalon, its upper surface pressed against the wall of the cranium, here 

 represented by connective tissue. Above is a ' plug ' of modified connective tissue, 

 above which, again, lie the dermis and epidermis. The complete absence of 

 pigment doubtless allows of the passage of light to the underlying sense organ. 



The ' parapineal organ ' lies immediately in front of the pineal instead of 

 beneath, as in Peti-mnyzon, its upper surface pressed against the fibrous cranium, 

 both structures upon dissection being visible from above. The parapineal lies 

 slightly to the left of the pineal organ, which tends to confirm the view that it 

 represents the left-hand member of an original pair. As in the European lampreys, 

 the parapineal is connected with the left habenular ganglion, while the pineal is 

 provided with a long nerve, passing backwards over the habenular ganglia and 

 connected with the posterior commissure. In the Petromyzon ammocoete Gaskell 

 holds that the nerve to the pineal eye is connected with the right ganglion, while 

 StudniSka seems inclined to think that it is connected with the posterior com- 

 missure alone, and therefore that the pineal and parapineal organs were originally 

 unpaired structures. In Geotria the pineal nerve is connected with both the 

 right ganglion and the posterior commissure, a fact which supports the older 

 view that the largely developed right habenular ganglion represents the ' optic 

 ganglion ' of the more perfectly developed i-ight pineal eye. 



The histological structure of the pineal agrees in the main with that described 

 by Studni6ka for Petromi/zon. The organ consists of a hollow vesicle with an 

 upper, transparent wall (the pellucida) and a lower, deeply pigmented retina, the 

 entire structure being obviously the swollen extremity of the elongated pineal 

 outgrowth, the proximal portion becoming solid and giving rise to the nerve. In 

 addition to the principal cavity there is, as in Petromyzon, an atrium, lined by 

 columnar epithelium and embedded in ganglion cells, and in Geotria completely 

 separated from the main cavity. The former connection between the two cavities 

 appears to be sometimes indicated by a funnel-shaped depression on the inner concave 

 surface of the retina. The retinal epithelium is composed of sensory and pigment 

 cells, the latter diff"ering from those of Petromyzon in that their inner ends are 

 segmented ofl" from the outer by what looks like a limiting membrane, which is 

 very conspicuous in depigmented sections. Careful observation, indeed, shows 

 that both pigment and sense cells are provided with difl'erentiated knob-like 

 inner extremities, those of the latter projecting into the cavity of the eye further 

 than those of the former. The cavity of the pineal eye is occupied by a network 

 of fibres, connected both with irregular projections of the pellucida and with the 

 knobs of the sense cells. 



Reissner's fibre, lately fully investigated by Sargent, is very conspicuous in 

 Geotria, where it can easily be traced from the central canal of the spinal cord, 

 through the fourth ventricle and iter, to the neighbourhood of the posterior com- 

 missure, beneath which it breaks up into fibrils, which are almost certainly con- 



