56 



Lumen 



Nachdruck verboten. 



A microscopic Study of tlie Umbilical Vesicle of a 13 mm 



hnmau Embryo, with special reference to the entodermal 



Tubules and the Blood Islands. 



By H. E. Jordan, A. M., Ph. D., 

 Associate professor of Anatomy, University of Virginia, U. S. A. 



V^ith 12 Figures. 

 (Schluß.) 

 The mesenchyme has at first a syncytial character (Figs. 10 and 11). 

 Large oval more or less irregular nuclei with pale staining nucleoli 



and a deep staining network with 

 net-knots lie scattered throughout 

 a delicately fibrillar and vacuolated 

 protoplasm. In such an area about 

 to become a blood island cell out- 

 lines appear in the mesenchyme. 

 Peripherally to such a mass the 

 nuclei of the undifferentiated mes- 

 enchyme arrange themselves in rows 

 (Fig. 10) and eventually form the 

 endothelial wall of developing blood 

 vessels. The nuclei of the mes- 

 enchymal syncytium are vesicular. 

 Immediately the cells arise, the 

 nuclei become smaller, stain more 

 intensely and have an oval or kidney- 

 shaped form (Fig. 11 b^). These 

 are the primitive blood cells or 

 hematogonia of Maximow. Their 

 offspring contain large spheric inten- 

 sely staining nuclei, and possess 

 a very narrow shell of basophile 

 cytoplasm (lymphocyte — Fig. 11 b^). 

 They are characteristically amoeboid 

 as seen by their pseudopodia (Fig. 12 b^). This cell or its offspring 

 rounds up and their nuclei become spheric and slightly less chromatic. 



«'— ^i 



Fig. 10. Drawiug of a blood island 

 of two cells (noi'moblasts) from the mes- 

 enchj^me of the older vesicle. The normo- 

 blasts appear capable of amoeboid motion. 

 The pseudopodia at the right seem to 

 merge into the connective tissue syn- 

 cytium. A capillary lumen is beginning 

 to form. Two stages in the metamor- 

 phosis of mesenchj'^me cells into "lympho- 

 cytes" (Maximow) are also shown. 

 X 1500. Reduced V4 ^^ reproduction. 



