228 CHARLES R. STOCKARD 



INTRODUCTION 



The origin of blood presents an almost unique problem in 

 embryologJ^ First, on account of the fact that the initial blood 

 anlage in many animals is contributed to by wandering cells. 

 Second, owing to the establishment of an early flow or circu- 

 lation of embryonic fluids before the blood corpuscles have 

 arisen. 



Soon after the cells and corpuscles are formed they are swept 

 into this circulating current and carried to all parts of the body. 

 In this way the blood cells become associated and mixed with 

 numerous other types of cells, and it is difficult, if not impossible, 

 to establish their true relationship with their surroundings. 

 For the above reasons one is often ready to believe that many of 

 the even careful and long thought out contributions to the devel- 

 opment of blood are, after all, largely a matter of the author's 

 own interpretation rather than a record of the actual processes. 



The general current of opinion at the present time would seem 

 to indicate that all blood cells arise from a mesenchymal type 

 of cell. A number of very competent workers have described 

 the change of this mesenchymal cell into a stem cell or mother 

 cell. On one side from this mother cell are developed various 

 leucocytes, which it is important to note always occur in an 

 interstitial position, while on the other hand, this same general 

 type of mother cell gives rise to other cells which later differ- 

 entiate into typical erythroblasts, and finally erythrocytes which 

 are always found to be located within the vessels. These so- 

 called indifferent mesenchymal cells probably, from the evidence 

 contained in the literature, do form blood cells, but to the dis- 

 criminating reader the evidence is not at all convincing that 

 both white blood cells and red blood cells really arise from one 

 common mother cell or common embryonic anlage. 



The possibility, and even probability, is certainly present that 

 these so-called stem mother cells may in reahty not all belong 

 to one type, but are different and may already be destined to 

 form either red cells or white cells. Yet on account of their 

 wandering capacities as well as on accoimt of the fact that the 

 earliest blood cells are swept around in the circulating current 



