2(38 PROCEEDINGS OF THE CANLDIAN INSTITUTE. 



the end-hud consequently corresponding in length only to the other 

 half. Otherwise with the sacs in question : the corium is hardly 

 disturbed by their presence : the bases of the epithelial cells which 

 form the fundus of the sac resting on it at the same level as the 

 ordinary palisade cells do. In preparations where the epidermis is 

 110 [i thick, the cavity of the sac is 80 fi deep, 18 ft wide in the ex- 

 panded fundus, and 6 /j. in the narrow neck. Whether the aperture 

 of the sac, which widens somewhat from the neck, be much larger 

 than it is broad (i.e., slit-like), in the stage in question, I am unable 

 to say, from the vertical sections at my disposal, but I am inclined to 

 think not. The walls of the sac vary in thickness from without in- 

 wards ; in the aperture the ordinary surface epidermal cells are found, 

 but the neck is bounded by cells, which are oval in outline where 

 they look into the cavity, (the long axis being disposed trans- 

 versely to the long axis of the sac), while their flattened opposite 

 ends converge downwards towards the corium, being imbricated 

 round the cells of the fundus like the scales of a bulb. The fun- 

 dus is occupied by a nerve-hillock, the neuro-epithelium of which is 

 quite similar to that in the ordinary canals, although, perhaps, only 

 three or four of the short sensory-cells may be counted in one section. 

 In my sections the hairs and bristles have not been preserved ; dif- 

 ferent methods of preparation would, of course, be necessary to de- 

 termine further the histological peculiarities of the sacs both in the- 

 youno' and adult. A.11 the cells that look into the sac, except those 

 of the neuro-epithelium, have a distinct cuticular border, which is 

 directly continuous with that of the superficial epidermal cells. In 

 still younger stages than that described the cavity of the sac com- 

 municates much more freely with the outside, and the characteris- 

 tic flask like shape has not yet been assumed. 



I have not studied the cutaneous ' nerve-sacs.' first discovered by 

 Leydig, which replace ordinary free nerve-hillocks on the head in 

 Ganoids, nor can I refer to Merkel's work in which these are accu- 

 rately described, but from the account (based on Merkel's) which 

 Wiedersheim furnishes of these, 1 I am inclined to believe that we 

 have here small 'nerve-sacs' of a similar character. It will be 

 observed, if the above description be compared with that which I 

 translate from Wiedersheim, that the agreement between the struc- 



» L. c, p. 361. 



