274 VERA DANCHAKOFF 



morphological structure and by well defined differentiation 

 potencies will be understood. 



The histogene wander .cells, which term I substituted by 

 histiotopic wander cells seem to be in close relation to the hemo- 

 cytoblasts and often are merely an intermediate but morphologi- 

 cally well defined stage of development between a mesenchymal 

 cell and a hemocytoblast. 



I wish to express my thanks to the director of The Wistar 

 Institute of Anatomy and Biology, Dr. M. J. Greenman, and to 

 the Staff of the Institute, for the generous hospitaUty shown to 

 me. I am indebted to Dr. S. Flexner for the kind admission 

 to the laboratories of the Rockefeller Institute for Medical 

 Research where the work has been partly done; to Dr. C. E. 

 McClung for the great interest shown in my work and for the 

 revision of a number of my preparations; to Dr. F. P. Mall for 

 the revision of the text and the proofs during my absence. 



3. HISTOGENESIS IN THE SPLEEN IN RELATION TO STRUCTURAL 

 ENVIRONMENT 



A. HISTOGENESIS IN A NORMAL CHICK SPLEEN 



The study of histogenesis in the spleen by the method of 

 stimulation of the stem cells in the spleen anlage requires a 

 thorough knowledge of the histogenetic processes, which nor- 

 mally take place in the spleen. The early stages of spleen devel- 

 opment in birds were studied by Tonkoff (41). Yet no investi- 

 gation of embryogenesis of the characteristic spleen elements 

 was made by modern technique. The general lines of differenti- 

 ation of the spleen elements were studied by Danchakoff ('16a) 

 in Tropidonotus natrix, but the question — what are the con- 

 ditions which determine the differentiation of the polyvalent 

 stem cell — remained unsolved. It seems, therefore, necessary 

 to make first a study of normal spleen development in the chick, 

 and to attempt to deterixdne the conditions w^hich imply the 

 various differentiation of the stem cells. 



The appearance of the spleen anlage in the chick embryo and 

 its first development corresponds to the description given by 



