206 BLOOD-DISCS OF REPTILES AND FISHES. 



there is considerable diversity as to the size of the discs ; but 

 the largest particles are found in the group of Amphibia, and 

 especially in those species which retain their gills through 

 life. The oval discs of Frogs (fig. 117) have a long diameter 

 of about 1-1 000th of an inch, and a transverse diameter of about 

 1-1 800th. Those of the perennibranchiate Amphibia ( 87) 

 may even be distinguished by the naked eye ; those of the 

 Siren having a long diameter of about 1 -435th of an inch, whilst 

 in the Proteus (fig. 118) the long diameter is stated occasionally 

 to reach 1 -337th of an inch. In FISHES, also, the size of the 

 blood-discs is variable ; they are 

 sometimes smaller (fig. 1 1 9), though 

 generally larger, than those of the 

 Frog ; but they never approach those 

 of the last-named remarkable ani- 

 mals. Hence the great size of the 

 Fig. ii9. BLOOD CORPUSCLES OP blood-discs of the curious Lcpido- 

 RoACH - siren (fig. 41) is strongly indicative 



a, a, b, red corpuscles ; c, colour- n f xi^ PprvHlinTi flffim'tiPQ nf fhnt 

 less corpuscle ; d, red corpuscle Ol tjie -ttepttUan affinities 01 tnat 

 treated with water. Species. 



231. It is by observing the large blood-discs of the Frog, 

 and still better those of the Proteus and Siren, that we can 

 obtain the best information as to their structure. They are 

 evidently fattened cells, having an envelope or cell-wall, which 

 consists of an extremely delicate membrane, and which con- 

 tains a fluid. The nucleus consists of an assemblage of minute 

 granules, which seem adherent to each other and to the wall 

 of the cell ; and it corresponds, in all essential particulars, to 

 the nuclei of the cells of other Animal tissues ( 32). The 

 fluid contained in the cells has a red colour ; and it is to this 

 that the peculiar hue of the blood of Vertebrata is owing. 

 When we are looking at a single layer of blood-discs, how- 

 ever, their red colour is not apparent, but they have rather 

 a yellowish tint; and it is only when we look through a 

 number at once, that the characteristic hue is seen. The 

 fluid is of about the same density as that in which the par- 

 ticles float; and thus neither will have a tendency to pass 

 towards the other. But, if we dilute the liquor sanguinis 

 with water, the fluid outside the cells will have a tendency 

 to pass towards their interior, according to the law of Endos- 

 mose. The cells will in consequence be first distended, and 



