ON THE BLOOD-CORPUSCLES OF THE TWO-TOED SLOTH. 69 



the smaller corpuscles of the camel neither power enabled us to de- 

 tect the presence of nuclei in the coloured blood-cells. 



Bearing in mind Nasse's observations 1 as to the comparative 

 frequency of the presence of a large colourless nucleus, or, in the 

 place of it, of an area of fainter colouration, in the coloured blood - 

 cells of the pregnant human subject, and also of the pregnant 

 bitch, I examined the blood from the uterine veins of a cow which 

 had been killed, in ignorance, as I was told, towards the end of 

 the period of gestation. But I was unable to discover any nucleated 

 red corpuscles in the blood from this source. 



It is well known that nucleated red blood-cells have been observed 

 in the blood of the human subject 2 , of the horse 3 , of the elephant 4 , 

 of the paco 5 , and of the sheep 6 ; but it should also be recollected 

 that round coloured blood-cells, so small as to resemble very closely 

 the normal mammalian blood, may be found very constantly in the 

 blood of certain ovipara 7 . Here, as in so many other cases, the 

 morphological value of a structural arrangement depends, not upon 

 an invariable presence or an invariable absence, but upon the con- 

 stancy of its quantitative preponderance. And upon this principle, 

 whatever other affinities to the sauroids the sloth may be supposed 

 to possess, the microscopy of its blood cannot be held to point in 

 that direction. That the red blood -cell — the carrier of oxygen, and 

 probably enough, the distributor of heat 8 generated in the body — 

 should present such different structural characters in the two classes, 

 Aves and Mammalia, which are both alike warm-blooded, is a fact of 

 the greater morphological importance for that it is physiologically 

 so hard to understand. From the purely anatomical point of view 

 it may be allowable to suggest that the enormous relative prepon- 

 derance of the lymphatic and lacteal gland system in the mam- 

 malia may account for the almost exclusive presence in their blood of 

 the small non-nucleated red blood-cell. 



1 Wagner's ' Handwbrterbuch,' i. 90, cit. M. Edwards, ' Lecons,' i. p. 66. 



2 Nasse, 1. c. ; Busk, • Quart. Journ. Mic. Soc.' 1852 ; translation of Kblliker's 'Hand- 

 book,' ii. p. 348, 1854. 



8 Wharton Jones, ' Phil. Trans.' for 1846, p. 73. 



4 Ibid., and Schultz, « Miiller's Archiv,' 1839, P- 2 5 2 > ^ ut see Cotti, ' Zeitschrift fur 

 Wiss. Zool.,' vol. v, cit. Kdlliker, 'Mikro-Anat.,' ii. 2, 583. 



5 Wharton Jones, 1. c. 6 Ibid. 



7 Funke, * Lehrbuch der Physiologie,' 4th edition, p. 213. 



8 Beale, in Todd and Bowman's • Physiological Anatomy,' p. 137. 



