104 



George Heuer. 



which Dr. Clark has shown are collapsed vessels capable of rapidly 

 expanding according to their state of functional activity. Hence 

 we may say that the border zone consists of terminal vessels either 

 collapsed or expanded. An injection mass may either open up 

 these ends or may rupture them. A considerable increase of pressure 

 will always rupture the terminal vessels of a border zone. The 

 best evidence that we have at present of these border zones of growing 

 lymphatics is in Dr. Clark's work in the living tadpole's tail where 

 one can see a row of growing tips beyond which there are no lymphatic 

 vessels whatever, only blood capillaries. The next best evidence 



Fig. 6. — Injected, lymphatics in the stomach wall of an embryo pig 6 cm. 

 long. The large mass is at the lesser curvature. 



comes in making numerous injections of succeeding stages and noting 

 that as the embryos increase in size the zone of injected lymphatics 

 spreads. 



Injections at 5.5 and 6 cm. show an increase in size of the retro- 

 peritoneal sac; for example, at 5.5 cm. the sac measures 7 mm. in 

 length, and there is a great increase in the number of ducts from it 

 and an extension of their zone. At 6 cm. lymphatic vessels have 

 extended far over the surface of the stomach and intestine (Fig. 6). 

 The retroperitoneal sac is being cut up into a mass of vessels 

 so that it has not the solid appearance in injections as at 3.5 

 and 4.5 cm. The breaking up of the sac into a plexus of 



