DEVELOPMENT OF WANDEEING MESENCHYMAL CELLS 161 



the time observed show ahnost all of its vessels loaded with blood 

 cells, and this is probably due to a slowing down gradually of 

 the circulation on account of the heart itself which finally stops 

 with the vessels in a balanced state. 



The study of the yolk-sac in the living embryo enables one to 

 observe every phase in the development of the red blood corpuscles 

 from the early time when they wander as amoeboid mesenchyme cells 

 to collect into groups of globular cells with short processes, the 'primi- 

 tive blood cells' of descriptive histologists , to be later surrounded by 

 vascular endothelium, and then to change from the globular wander- 

 ing cells into the flattened ellipsoidal erythrocyte loaded with 

 haemoglobin, and finally freely floating in the current of the blood 

 stream. The fully formed corpuscles apparently become incap- 

 able of independently migrating even when not carried by the 

 circulation. 



DISCUSSION AND CONCLUSIONS 



In the previous paper on the origin of blood and endothelium, 

 a somewhat full discussion of the problems of blood formation 

 in the teleosts and other vertebrates was entered into. A con- 

 sideration of the questions of origin and development of vascular 

 endothehum was also undertaken in the light of the results there 

 presented and the more recent general literature bearing on this 

 subject. The experimental results then contributed seemed 

 in the light of the past literature to render highly probable, if 

 not to actually prove, the polyphyletic origin of the various types 

 of blood cells, as opposed to the now extrernely improbable 

 monophyletic theory of origin of blood cells and vascular endo- 

 thelium. For a general consideration, the reader is referred 

 back to these discussions. 



A number of particularly significant points are brought out in 

 the present study of the living normal and experimental embryos 

 which bear directly on several of the past theories and specu- 

 lations regarding the origin of vessels and blood. Only these 

 special points will be briefly considered and analyzed at this 

 time. 



