of a cell type or stage of development because, 

 as already mentioned, immature cells when they 

 are carried by the circulating blood often have 

 an appearance that is different from that pre- 

 sented when they are in the environment of the 

 organ from which they came. 



A distinction has been made between abnor- 

 mal cells and artifacts; actually there is a third 

 category, namely, variations from the typically 

 normal, and to place each atypical cell in the 

 correct one of these three categories has taken 

 far more study and experimentation than find- 

 ing, illustrating, and describing the "typical" 

 cells. Likewise, the possibility of error is 

 greater, so that future research may well discover 

 that what has been called abnormal is, in reality, 

 an artifact, or merely a variation of the normal. 

 Cells are called abnormal on the assumption that 

 the abnormality in them was present within the 

 body of the bird, and artifacts are considered to 

 be deviations from the normal that presumably 

 were caused in the process of taking blood, or 

 drying it, or applying the stain. Since so much 

 less is known of the cytology of immature blood 

 cells than of the mature types, the series of ex- 



amples showing abnormalities and artifacts are 

 much fewer in the chapters on embryo blood and 

 on bone marrow cells than in chapter 2. 



Chapters 3, 4, and 5 have much in common 

 in regard to their subject matter; they all deal 

 with cells during development. This is true, 

 whether the cells were collected from functional 

 circulating blood of the embryo or from various 

 hematopoietic organs of embryo or adult. 



Chapter 6 is devoted primarily to a descrip- 

 tion of blood cells of avian species other than 

 the chicken. It was soon discovered that sim- 

 ilarities in the morphology of blood cells of 

 different species were much greater than 

 differences. Included also in this chapter is tab- 

 ular material on cell sizes and cell counts. 



A chapter on technic was included at the end 

 of the volume. Sometimes it happens that seem- 

 ing differences in cell morphology can be traced 

 to the use of different technics by different inves- 

 tigators. It was hoped that this difficulty might 

 be avoided if the technics used in this study 

 were set forth. Many methods in addition to 

 those discussed here have been applied to avian 

 blood studies. 



16 



