ORIGIN OF BLOOD AND ENDOTHELIUM 231 



The bony fish is important as an object of study on account 

 of the fact that so many of its organs and tissues arise in a way 

 peculiar to the group and differing from the other vertebrate 

 classes. The sohd gastrular invagination described by Sumner 

 ('00), the original solid condition of the central nervous system, 

 the solid optic knob which changes into the optic vesicle, and 

 in the present connection, the very particularly interesting solid 

 cord of cells, the intermediate cell mass, which is to give rise to 

 the red blood corpuscles of the individual make the Teleosts a 

 group of great embryological interest. 



The complexity of the problem concerning the origin of the 

 various types of blood cells is then largely due to the migration 

 and mixture of the cells involved. It is strange that up to now 

 no investigator has attempted in an experimental way to analyze 

 the situation. It would seem to be one of the most favorable 

 problems for an experimental analysis, and in the end it is cer- 

 tainly an analytical problem. 



If it were possible by any means to separate the anlage of the 

 red blood cells from that of the white blood cells and prevent 

 the flow of fluid in the embryonic body so that these cells would 

 not frequentl}'- become intermixed, then it would seem possible 

 to determine clearly the entire genesis of the various type cells. 

 If all the types of blood corpuscles did arise from a common 

 mother mesenchymal cell they should then be found in intimate 

 association throughout all blood forming regions. Further, if 

 the vascular endothelium really has blood forming power, it 

 should be found that blood cells arise in any region of the em- 

 bryo which possesses vessels lined by such endothelium. 



There have been various experiments performed which have 

 interfered more or less with the circulation of the body fluids 

 of the embryo, but none of these experiments where aimed at 

 a solution of the genesis of blood cells or have been used for such 

 a purpose. Knower ('07) removed the heart anlage from early 

 frog embryos and they continued to develop in some cases with 

 almost no circulation. In other specimens there was a very 

 feeble sluggish circulation due to the pulsation of the lymph 



