29G Herbert M. Evans. 



Eleven of these vessels spring from the lateral aortic wall and anas- 

 tomosing, form a simple plexus in the limh. Six venules may be 

 seen entering the posterior cardinal trunk. The umbilical vein, 

 seen faintly in the somatopleure, is of considerable size and receives 

 many tributaries from the limb (Fig. 5). 



It is well to call attention here to a constant phenomenon observed 

 in the spread of the capillaries through the limb bud. They extend 

 in every direction through the limb tissue, and fill it with a uni- 

 form mesh of vessels, save in a definite border zone. This border 

 zone or marginal non-vascular^- area is never invaded by. capillary 

 sprouts and remains uninvaded till the time of establishment of 

 the border vein. The latter structure, constructed out of the most 

 peripheral portion of the limb's plexus, thus marks the old boundary 

 between the primary vascular and non-vascular zones. 



Embryos 5 and 6 (Figs. 6 and 7) furnish other instances of the 

 variations in the exact pattern of these capillaries forming the pri- 

 mary limb plexus. Both the anastomosis of the subclavian capilla- 

 ries soon after their emergence from the aortic wall, and the occa- 

 sional division of these vessels soon after their origin, are to be ex- 

 pected from the usual behavior of capillaries. Instances of this are 

 seen in both figures. Some writers have seen fit to especially remark 



^^The significance of non-vascnlar areas is as yet unsolved. However, 

 careful studies on a series of complete injections show them to be a definite 

 feature in the circulation of every region of the early embryo. We have 

 to do here perhaps with a matter of cell chemistry and tropisms, for endo- 

 thelium apparently avoids certain areas in the embryo — the non-vascular 

 areas. In the case of the arm buds, the early non-vascular area consists 

 of a narrow strip of denser mesenchyme adjoining the ectoderm. 



Fig. 5. — Chick embryo of sixty hours incubation (embryo 4 of table), show- 

 ing profuse outgrowth of primary subclavian capillaries into early limb bud, 

 X 53%. 



14th D. I. v.. fourteenth dorsal intersegmental vein, that of the fifteenth 

 iuter-somitic space. 



Prim. Cap. I'lexus, primary capillary plexus of the limb bud. 



Post. Card. Vein, posterior cardinal vein. 



Prim. Subcl. Art., primary subclavian artery of left side, the lowest one of 

 four subclavians here, but opposite the seventeenth dorsal inter-segmental 

 vessels and already the largest of the series. 



