214 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



that are at least one and a half times as broad as a normal 

 corpuscle ; they may be much larger. 



It is important to learn to recognise them, since their presence 

 is an almost constant sign of pernicious anaemia, and it is rare 

 to find them in any considerable numbers in other conditions. 

 When you are frequently examining blood-films with the same 

 lens and eyepiece, you will soon be able to pick out forms of 

 abnormal size. When you have not had this experience it is 

 advisable to make a film of normal blood for comparison, and to 

 look first at one and then at the other. A still better method 

 (due to Shattock) is to mount the two films (the normal and 

 abnormal, both stained) face to face, and then to mount the pair 

 on a slide, and examine them with an oil-immersion lens ; a slight 

 turn of the fine adjustment will enable you to pass at once from the 

 one to the other, and to compare the diameters of the corpuscles 

 with some degree of accuracy. Another useful method is to 

 employ the counting-chamber of the haemocytometer, and an 

 idea of the diameter of the reds should be gained in all cases of 

 pernicious anaemia whilst the count is being made. The diameter 

 of one of the small squares is almost exactly six times that of a 

 normal red corpuscle. If you look at the squares with a central 

 ruling, you will be able to compare any corpuscles which may be 

 in them with the semi-diameter ; the one should be one-third of 

 the other, and you may consider as a megalocyte any corpuscle 

 which is one-half or more of this semi-diameter. 



3. PoikilocyUs (Plate VIII., Fig. 6) are deformed corpuscles, 

 and are typically shaped like a pear, but may be kidney-shaped 

 or quite irregular. They may be about as large as a normal 

 corpuscle, or smaller, or larger, and they may stain abnormally. 

 Poikilocytes are more common in pernicious anasmia than in 

 other diseases, but are not of much diagnostic importance, since 

 they only occur in advanced stages of the disease, long after the 

 diagnosis should have been made. To recognise them, put a 

 small drop of blood on a perfectly clean slide, apply a cover-glass, 

 and examine at once ; do not identify poikilocytes in dried films 

 or in the counting-chamber of the haemocytometer until you have 

 had a good deal of experience, as accidentally injured and con- 

 torted forms may occur in either. 



4. Polychromatophil Degeneration (Plate VIII., Fig. 3). — In this 

 condition the corpuscle (which may otherwise be normal or 

 abnormal in other ways), instead of being strictly acidophile in 



