PROCURING OBJECTS. 95 



nucleus or spot in the centre, and in the other a thickened 

 edge, like a ring (a, Fig. 28). In all air-breathing, oviparous, 

 vertebrated animals, the blood-corpuscles are oval, and a nu- 

 cleus may be observed within each of them. This nucleus is 

 rendered very distinct by the addition of a drop of diluted acetic 

 acid. 



The observations of Professor Owen on the blood-discs of 

 the Siren acertina, b, Fig. 28, show that the nucleus consists 

 of a cluster of nucleoli enclosed in a capsule in the centre of 

 the oval blood-disc. The length of the disc in the Siren is 

 ^Jflth of an inch, while the diameter of human blood-discs 

 average -3^ 0th of an inch. 



Fig. 28. 



Very frequently, under the microscope, the blood-corpuscles 

 unite by their flat surfaces, so as to form rows, like piles of 

 coin ; the disposition to which is proportionate to the quantity 

 of fibrin in the blood. 



When the corpuscles are observed in a drop of blood spread 

 out between two plates of thin glass, they will often be seen to 

 present a tuberculated or mulberry appearance, which is sup- 

 posed by Donne to depend upon commencing desiccation, and to 

 arise from deficiency of serum. Others ascribe it to evapora- 

 tion from the edge of the slide. In many of my own observa- 



