ON THE BLOOD-DISCS OF SIREN LACERTINA. 75 



regular than the inner one, which appeared crenated. This appearance 

 was due to the structure of the nucleus, or the contents of the nucle- 

 olar capsule, which was indicated by the outer line. These contents 

 consisted, in every blood-disc examined, of a number of moderately 

 bright spherical nucleoli sufficiently distinct to be counted, when viewed 

 by a Powell's -^th ^ ncn objective, with the eye-piece magnifying 700 

 linear diameters ; the ordinary number of nucleoli seen in one plane or 

 focus being from twenty to thirty ; the total number was of course 

 much greater. The facility, as well as certainty of the demonstration 

 of such a structure in a good microscope of the present day, will be 

 readily admitted, when it is remembered, that the nucleus of the blood- 

 disc of the Siren is three times the size of the entire human blood- 

 disc. These tuberculate nuclei, when removed from the capsule, were 

 colourless ; the component granules or cells have a high refractive 

 power ; viewed in situ, they present a tinge of colour lighter than that 

 of the surrounding fluid interposed between the nucleus and the 

 capsule. 



The external capsule of the blood- disc is smooth, moderately resist- 

 ing, elastic, as was easily seen by the flattening of the parts of two 

 blood- discs that might come in contact, and the recovery of form when 

 they were floated apart. 



As the fluid contents of the blood-disc in part evaporated during the 

 process of desiccation, the capsule fell into folds in the interspace be- 

 tween the nucleus and the outer contour, these folds generally taking 

 the direction of straight lines, three to seven in number, radiating from 

 the nucleus. 



Explanation of the Figures, Plate I., Division 2. 



Blood-discs of Man and Siren, drawn by the Camera lucida, under a magnifying 

 power of 700 linear dimensions. ' 



3, Human blood-discs. 4, Ditto, viewed edgewise. (The upper portion of this 

 figure has been drawn too wide ; it should be the same as the lower part, 2, Siren's 

 blood-disc. 1, Ditto, viewed edgewise, a, Folds of external capsule, produced 

 by desiccation, b, c, Nucleoli; the capsule of the nucleus, seen enclosing the 

 nucleoli. 



