74 ON THE BLOOD-DISCS OF SIREN LACERTINA. 



The absolute size of these particles in that perennibranchiate reptile, 

 in which they may be distinguished by the naked eye, renders them 

 peculiarly adapted for minute investigations into the structure of the 

 nucleus and capsule of the blood-disc ; but the value of the relation 

 between their size and the persistency of the external gills, must 

 depend upon the correspondence of other perennibranchiate reptiles 

 with the Proteus in this respect. The superior size of the blood- 

 discs of the newts to those of the land salamanders and tailless Ba- 

 trachians has been confirmed by Professor Van der Hoeven's obser- 

 vations on the blood-discs of the gigantic newt of Japan, (Sieboldtia, 

 Salamandridae, Vol. xx, p.p. 331, 332), of which a fine specimen has 

 been for several years kept at Leyden ; and I have been able to add 

 another instance to the still greater relative size of the blood discs in the 

 perennibranchiate reptiles, by the examination of those of the largest 

 existing species of that family, the Siren lacertina, of w r hich a specimen, 

 twenty inches in length, is now (Oct. loth, 1841) living at the Zoolo- 

 gical Gardens. The blood was obtained from one of the external gills, 

 and immediately subjected to examination. The blood-discs presented 

 the eliptical form which hitherto, without exception, has been found to 

 prevail among the air-breathing oviparous vertebrated animals ; the 

 ellipse was not quite regular in all the blood-discs ; several were sub- 

 ovate, a few slightly reniform and thicker at the more convex side ; all 

 were as compressed or disc-shaped, as in other Batrachians, with the 

 nucleus slightly projecting from each of the flattened surfaces. 



The nucleus did not partake in the same degree with these varieties 

 of form, but maintained a more regular elliptical form ; the varieties 

 in question appearing to depend on pressure acting upon the capsule 

 and the coloured fluid surrounding the nucleus. Yet when the ellipse 

 of the blood-disc was, as it happened in a few cases to be, longer and 

 narrower than the average, the form of the nucleus presented a similar 

 modification of size. 



The following is a table of the averages of many admeasurements of 

 these blood-discs, made with one of Powell's screw micrometers : 



Long diameter zi~o tn f an English inch. 



Short diameter -8To th to T<) th 



Long diameter of the nucleus 10 1 00 th " 



Short diameter of ditto W^o th 



Thickness of ditto arVo 1 ^ 1 " 



(As viewed edgeways covered by the capsule.) 



The nucleus was circumscribed by a double line, the outer one more 



