460 ON THE ORIGIN AND EARLY DEVELOPMENT 



LYMPHATICS OF POSTERIOR LYMPH-HEART REGION IN CHICKS OF 4 TO 5 DAYS. 



As stated above, the plexus of blood-filled lymphatics in the region of the 

 posterior lymph-heart, and in the more superficial regions ventral and anterior to it, is 

 easily visible in living chicks of 5 days. Pushing the study of this region back to still 

 earlier stages, we found the first evidence of lymphatics in the living in embryos of 

 about 12 mm. greatest length (measured in the fresh) and about 4 days 20 hours to 5 

 days old. These first lymphatics were visible as a number of separate knobs of stag- 

 nant blood in the region just lateral to several of the dorsal intersegmental coccygeal 

 veins. At this stage these little dots of blood give the region of the posterior lymph- 

 heart a characteristic speckled appearance. Since the knobs lie between the observer 

 and the veins their connections can not be seen, but ink granules injected directly 

 into them can be observed to pass into the nearest intersegmental coccygeal vein. 



In many cases we studied this region in the living 4 to 8 hours before the appear- 

 ance of these knobs, and were satisfied that no circulating-blood vessels are present 

 in the exact area where these vessels make their appearance. About an hour after 

 the first knobs become visible new ones can be seen near them, and connected by 

 narrow vessels. Injection of these structures shows small, discrete clusters, somewhat 

 resembling bunches of grapes, connected, as were the earlier knobs, with the inter- 

 segmental veins of the tail. The steady and rapid extension of these blood-filled 

 structures to form a plexus was observed in many embryos. They extend toward 

 the surface and spread out in the region superficial and ventral to the lymph-heart 

 region. Two or three delicate projections from the plexus can first be seen, connec- 

 tions between these make their appearance, then certain parts of the plexus enlarge 

 and become more densely packed with blood-cells. New sprouts appear in advance, 

 and the same process of extension, accompanied by plexus formation, is repeated. 

 The several parts of the plexus are irregular in size; many of the lymphatics are 

 several times as wide as a blood-capillary, while some of the connections and pro- 

 cesses are as small as and often much smaller than a blood-capillary. All of these 

 successive stages were tested by pressure over the parts filled with blood, in order 

 to see if the blood could be forced farther, and also tested by injection. By both of 

 these methods it was found that, in these early stages, practically all of the injectible 

 lymphatics are normally filled with stagnant blood. This blood, which backs up 

 from the veins with which the early lymphatics connect, forms a vital injection mass 

 which is constantly being forced into the developing plexus, and by this means 

 reveals the extension of the continuous lumen of the lymphatics. 



As already stated, in chicks of 5 days, sectioned parallel to the surface, a plexus 

 of delicate vessels could be reconstructed, by use of oil-immersion, well beyond the 

 limit of the injectible or blood-filled lymphatics. We next studied still younger 

 embryos by this method of paper reconstruction, with the aid of the oil-immersion, 

 in an effort to trace back the picture of the developing lymphatics. 



Plate 5 is a reconstruction of the region of the posterior lymph-heart in a chick 

 of 4 days 23 hours, measuring 12 mm. in the fresh state. In this specimen, examined 

 in the living, the first knobs filled with blood were visible. They were not injected, 

 but from other specimens of the same stage it may safely be inferred that injections 

 would have shown us small clusters connected with the intersegmental coccygeal 



