428 KANDOLPH WEST 



derm, just lateral to the coccygeal veins. Two or three lateral 

 branches of each coccygeal vein (fig. 12, 9) pierce the muscle plate 

 and proceeding directly outward terminate in a plexus of haemal 

 capillaries which lie directly beneath the ectoderm. 



The lymphatic plexus, which later forms the lymph heart, 

 develops in the mesenchyme between the caudal muscle plate 

 and this superficial plexus of haemal capillaries. A nmnber of 

 isolated spaces, bounded by mesenchyme cells which are still 

 practically unflattened, are seen (fig. 12, S; fig. 5, 8) to occupy 

 the position just alluded to. They have been studied very 

 carefully with oil iniinersion lenses and are absolutely inde- 

 pendent of any vascular connection, either v/ith the lateral 

 branches of the coccygeal veins or the haemal capillaries; they 

 occur only caudal to the level of the hind limb bud and only 

 lateral to the muscle plate. 



Chick of 14 mm. Reconstruction of the blood vessels of the 

 caudal region, and the lymphatic plexus in so far as it forms a 

 continuous channel connected with the veins. X 150. Figure 13: 

 Arteries black, veins and capillaries white, lyniphatics connected 

 with veins, green. The isolated spaces have been omitted from 

 this reconstruction in order that the lymphatic plexus connected 

 with the veins might be more clearly shown. The reconstruc- 

 tion has been drawn from the side and somewhat from above 

 and the aorta and postcardinals have been shown in the draw- 

 ing as folded upward and outward. We have in this recon- 

 struction practically the same arrangement of a.rteries, veins 

 and haemal capillaries as was described for the 11 mm. embryo. 

 The two postcardinal veins (fig. 13, 18) are seen above and some- 

 what lateral to the aorta; they anastomose above that vessel, 

 and receive the coccygeal veins both cranial and caudal to their 

 anastomosis. The coccygeal veins (fig. 13, 3) as before, pass 

 downward, at right angles to the axis of the vertebral colu nn, 

 close to the caudal muscle plate, and give off lateral branches 

 (fig. 13, 9 a, b) which pierce the muscle plate. It will be seen 

 that a plexus of lymi)hatic vessels connected with the coccygeal 

 veins has been established between the haemal capillaries and 

 the muscle plate, which is chai-acterized by the irregular size 



