drawings were made with a 90X objective and 

 12.5 X ocular. During the engraving process, 

 all the low-power drawings thai originally were 

 approximately 3 x 3% inches were increased in 

 size 50 percent, so that, as presented here, they 

 are 4^/2 x SWie inches or slightly more. There- 

 fore, in the low-power drawings, the cells are 

 shown at about 1370 times their natural size. 

 The purpose of the low-power drawings is to 

 give the overall impression that one has when 

 looking through the microscope and ojjsei-ving 

 different kinds of cells in the same field. Under 

 such conditions one can distinguish minute dif- 

 ferences in color, tone, or texture that often van- 

 ish when single cells are removed from the en- 

 vironment of other cells, even when they have 

 been drawn at a much higher magnification. The 

 low-power drawings serve another purpose in that 

 they contain about four times as many cells as 

 are represented in the high-power drawings; 

 therefore, the same cell type shown at high mag- 

 nification can be presented sufficiently often so 

 that deviations from the typical are fully illus- 

 trated. If all these variations were presented as 

 high-power drawings the Atlas would be unduly 

 large. 



An outline of the individual cells of each low- 

 power drawing is given on the facing page along 

 witli the legend. Identification of the cells in the 

 field is made by numbers placed on or near the 

 cells of the outline drawing. 



Blood from mammals often spreads unevenly 

 over the slide when the smear is made and in par- 

 ticular shows a clumping of platelets and an 

 aggregation of leukocytes along the margins of 

 the preparation. In order to reduce this tend- 

 ency as much as possible, the mammalian hema- 

 tologist has often used coverglass smears. This 

 has not been necessary for preparations of avian 

 blood because the cells in the average well-made 

 smear do not segregate, there is no clumping of 

 thrombocytes, and the erythrocytes do not rear- 

 range themselves in rouleaux formation. The 

 simplest and easiest method is to place a small 

 drop of blood on the end of a "pusher" slide and 

 to touch this to one end of another slide where it 

 is held for a moment — long enough for the drop 

 to spread to each edge of the pusher slide. The 

 latter slide is then steadily and rather quickly 

 slid to the opposite end of the "smear" slide at 

 an angle of about 45 degrees. 



The appearance of the typical blood smear in 



which diere is a relatively even distribution of 

 cells is obtained from all hatched chickens, ex- 

 cept laying hens, as shown in figure 1 F. The 

 scratches and abraded spots that often come when 

 the slides are blotted or handled roughly are 

 nicluded. The appearance of a smear made from 

 the blood of a laying hen is shown in figure 1 G; 

 the fat globules in the senmi spread when the 

 smear was made and pushed the cells of the un- 

 dried layer aside. This, however, disturbed 

 only slightly the uniformity of distribution of 

 cells and did not cause certain cell types to 

 segregate. 



ARRANGEMENT OF SUBJECT MATTER 



It is often helpful to the reader of a scientific 

 liook if the writer reviews briefly the general plan 

 of organization and what was in his mind when 

 seemingly unrelated things sometimes were 

 placed beside each other. In this study, the list 

 of chapter headings reflects the scope and se- 

 cpience of the fields covered. 



More emphasis has been placed on the subject 

 matter of the second chapter than on any other. 

 It contains almost as many drawings as do all 

 other chapters combined. The purpose of this 

 emphasis is to give as nmch help as possible to 

 the field worker in poultry diseases, whose first 

 consideration when confronted with an unknown 

 condition is to arrive as quickly as possible at a 

 preliminary diagnosis. 



The four categories into which the study of 

 each cell type is grouped is best exemplified in 

 the erythrocyte series from the circulating blood 

 of the hatched chicken. These series fall into 

 the following classifications: (1) Normal mature 

 cells, (2) normal immature forms found in cir- 

 culating blood, (3) abnoi-mal cells, and (4) arti- 

 facts. Sometimes the variety of immature cells 

 found in circulating blood is so great that these 

 cells appear to represent the complete develop- 

 mental series, but they were included under the 

 circulating blood to indicate the range of cell 

 types that might be picked up in a general ex- 

 amination. In the study of embryo circulating 

 blood and blood from hematopoietic organs, 

 these cells are shown in their proper setting in 

 the form of a complete series. This may appear 

 to be a duplication of effort, but it has proved 

 to be of great help in the exact characterization 



15 



