BLOOD 



" blood islands " of the extra-embryonic vascular area of the meso- 

 blast. Their mode of origin in this location is essentially intra- 

 cellular, through the medium of the " vow-formative cells." This 



process consists essential- 

 ly in the enlargement and 

 7i excavation of the vaso- 

 formative cell groups, 

 their nuclei undergoing 

 division by mitosis, some- 

 times without corre- 

 sponding division of the 

 cytoplasm, the daughter 

 nuclei forming in some 

 cases primitive nucleated 

 blood cells, and in others 

 the nuclei of the endo- 

 thelial wall. According 

 to Ranvier, minute par- 

 ticles of the vaso-forma- 

 tive cytoplasm may sepa- 

 rate from the parent 

 mass to float free as non- 

 nucleated, hemoglobin containing cells within the primitive blood 

 vessel. 



Similar vaso-formative processes occur throughout the meso- 

 blastic tissues in the early periods of fetal life. They have been 

 carefully studied in the subcutaneous tissue and the omentum by 

 Schafer, Ranvier, Minot, Nicolaides, et als. ; but more recent ob- 

 servations by S. Mayer, E. Neumann, Spuler and Fuchs tend to 

 throw discredit upon the previous conclusions, inasmuch as these 



FIG. 76. "VASO-FORMATIVE" CELLS FROM THE MES- 

 ENTERY OF A BABBIT SEVEN DAYS OLD. 



gr.s., red blood cells; w, nucleus of the vascular 

 endothelium ; p, points of growth, at which extension 

 occurs. Highly magnified. (After Eanvier.) 



FlG. 77. A " VASO-FORMATIVE CELL. 7 



a, isolated red blood cell. Note the apparent disintegration of the red blood cells shown 

 in the middle of the figure. (After Fuchs.) 



observers maintain that the vaso-formative cells instead of being 

 developing cells are in reality in a state of degeneration, they hav- 

 ing been separated from the general vascular current by occlusion 

 of the lumen. The small, hemoglobin containing, protoplasmic 



