TRANSACTIONS OF SECTION D. 605 



nected with the epithelium of the ependymal groove, as described by Sargent for 

 Petromyzon, It is easy to trace some of the fibres of the pineal nerve to the 

 inner side of the same epithelium, through which they are possibly connected 

 with Reissner's fibre. 



My observations in the case of Sphenodon are as yet incomplete, but certain 

 remarkable and hitherto unrecorded features have come to light. As in Geotria 

 and Petromyzon, the sense cells have the form of unpigmented rods, whose slender 

 apices project into the cavity of the eye, and are there connected with a network 

 of fibres, but without being appreciably enlarged. Between the sense cells lie 

 irregular elongated masses of dark-brown pigment granules, no pigment cells 

 having been yet recognised. Behind the layer of pigment and sense cells lies a 

 layer of nerve fibrils and ganglion cells, backed by another layer of doubtful 

 significance. The lens is almost completely disconnected from the retina. 

 Histologically it exhibits great peculiarities, the most remarkable of which is the 

 presence of a large ' central cell,' irregular in shape, with a conspicuous nucleus, 

 and looking like a large ganglion cell. 



5. The Birds and Mammals of Yorkshire. By Oxley Grabham. 



TUESDAY, AUGUST!. 

 The following Papers were read : — 



1. Spicule Formation. By Professor E. A. Minchin, M.A. 



A spicule is to be defined as a sclerite of intra-cellular origin and growth. It 

 may be formed of organic material entirely (example, acanthin spicules of Radio- 

 laria), or the organic material may become impregnated with, or even entirely 

 replaced by, mineral salts. The interest attaching to the study of spicules lies in the 

 fact that an inorganic body is thus formed in the midst of the living substance, and 

 that the material composing the spicule may be subject to physical laws or exhibit 

 peculiar properties arising from its inherent molecular constitution. Thus in 

 calcareous sponges each spicule, whatever its form, is always a single crystal of 

 calcite, but the form of the spicule is always quite diflferent from that which the 

 calcite crystal normally assumes when formed in non-living surroundings, so that 

 the crystalline nature of the spicule could not be suspected from simple inspection. 

 Hence spicules represent a meeting-ground of organic and inorganic forces. 

 What part is played by each of these factors in determining the form of the 

 spicule ? Do the vital activities alone assert their sway, or are the inorganic 

 forces able to modify the joint product ? 



In calcareous sponges only three forms of spicule are foimd (i) Monaxon, simple 

 needle-like forms with one end embedded in the tissues, and the other projecting 

 into the water; (ii) Triradiate, i.e., three rays joined at a centre, and completely- 

 embedded in the sponge wall ; (iii) Quadriradiafe, i.e., triradiate, with a fourth 

 ray joined to the other three at or near the centre and projecting into the interior 

 of the sponge. There are, further, two distinct types of triradiates, characterising 

 different sections or families of the Calcarea — (1) equiangular, in which the three 

 rays meet at equal angles of 120°; (2) sagittal, in which there is always an 

 unpaired angle greater than 120° and two paired angles less than 120°. 



Primary spicules, derived each from a single mother-cell, must be distinguished 

 from secondary spicules or spicular systems, derived from more than one mother- 

 cell. In Leucosolenia each monaxon is a primary spicule, whereas the triradiates 

 are secondary, each ray being formed in the same way as a monaxon. Each 

 tru-adiate is thus to be interpreted as a system of three primary spicules joined 

 together. In the quadriradiate, the fourth ray, from its formation, is to be 

 regarded as a primary spicule fused to the triradiate system. 



