376 The Development of the Lymj)hatic System 



mose between the scapiilaB when the pig is 3 cm. long, so that the 

 border zone is early obliterated here. Figure 3 shows that in the 

 border zone the ducts grow out in advance of the plexus. In inject- 

 ing, when the pressure is increased, these advance ducts always burst 

 at the rounded tips, showing that they are the ends of the ducts. If 

 now the ducts are just filled, care being taken not to burst them and 

 each duct is touched just at the point where it leaves the plexus, with a 

 small glass rod, it will be noted that the duct expands and contracts as 

 the pressure is varied, or in other words, the wall is continuous and 

 elastic. Occasionally it happens that one of these blunt ends, which 

 may even be bulbous, is not really the end of a duct, for by a little 

 pressure the injecting fluid can be forced out into a long thread-like 

 process. That this is really a duct is shown by the fact that it can be 

 iilled and refilled by varying the pressure. These fine ducts or sprouts 

 represent the process of growth at the border zone. In areas where 

 the ducts are growing more rapidly than they do over the side of the 

 head the terminal ducts are smaller than those shown in Fig. 2, and 

 almost every one will have one or more long sprouts running out in 

 advance. 



It has already been said that no lymphatics have ever been injected 

 in the areas beyond these zones of lymphatics in the different stages. 

 Beside these injection experiments to prove that the ducts in the border 

 zone are terminal and that no lymphatics can be injected beyond, com- 

 plete serial sections have been made showing, first, the zone itself, with 

 its rich capillary plexus, second, the zone of the growing tips, and 

 third, the areas not yet invaded by lymphatics. These early ducts are 

 so large that there is no mistaking them in sections; they are many 

 times the size of blood capillaries. Uninjected specimens stained in 

 acid fuchsin show them especially well in contrast with the blood 

 capillaries. 



I now considered the point proved that the lymphatics gradually 

 invade the skin, for by making injections from either of the two radiat- 

 ing points in successive stages one can inject a wider area as larger pigs 

 are taken and around these areas there is always a border zone of 

 terminal ducts, which burst at the tips if pressure is used. The tips 

 of these ducts are growing points and often have sprouts running out 

 from them and finally, beyond this zone, there are no lymphatics, as 

 has been proven both by their absence in sections and by a large num- 

 ber of negative injection experiments. 



The next step in the growth of the lymphatics was to find out how 

 they reached the surface. This was studied first in the neck. The 



