Florence R. Sabin 371 



Up to the time when the pig embryo reaches the fish stage, that is 

 to say, when the four visceral arches are plainly seen (see Keibel's 

 Normaltafeln, 1. Das Schwein., Fig. 19), there are no lymphatics. 

 This is true of embryos up to 14 mm. long, which corresponds to a 

 human embryo of about five weeks. There are, however, in these early 

 stages certain areas in which loose connective tissue, bounded by zones 

 of denser tissue, forms channels of least resistance for fluid injected 

 under pressure. For example, there is such an area around the central 

 nervous system. If Prussian blue is injected just dorsal to the spinal 

 cord near the tail it will not only outline the cord and the brain but 

 will also surround the peripheral nerves as far as they have developed. 

 Sections of such specimens, especially if thick, give deceptive pictures, 

 for the blue granules lying in the meshes of the connective tissue look 

 as if they were in definite ducts. This is, however, not the case, and 

 though these wide intercellular spaces, being full of lymph, may be 

 called lymph spaces, and may have an important relation to the nour- 

 ishment of the nervous system, they 'are not a part of the lymphatic 

 system. In sections this loose tissue often breaks away, especially 

 around the nerves, and gives the false appearafice of empty spaces. 1^ 

 is in a similar way possible to outline the Wolffian body at least in part 

 by injection. 



Another of these areas of loose tissue bounded by zones of denser 

 tissue is found beneath the skin. If one injects Prussian blue into the 

 tissue beneath the skin of the embryo pig, there will be at the point 

 of injection a mass of the blue fluid from which straight, blunt pro- 

 cesses reminding one of Budge's canals, run out often in parallel lines. 

 These processes have no resemblance to the true lypmhatic ducts which 

 lie at a more superficial level and can be injected over them, but are 

 due simply to the separation of the connective-tissue cells and show 

 that the intercellular spaces are lines of least resistance for fluid in- 

 jected under pressure. These spaces are artificially widened by injec- 

 tion. The distance one can inject these spaces depends on the loose- 

 ness of the connective tissue, and as the Aeshes of the connective tissue 

 are widest around the central nervous system, it is here that one can 

 inject the farthest. 



Serial sections of several embryos of stages before the lymphatic 

 system has begun, that is of pigs up to 14 mm. long, have been made. 

 In these specimens the blood-vessels are injected and from a study of 

 the sections it is clear that all the spaces in the body walls can be 

 proven to be blood-vessels except the spaces between the individual 

 cells. There are no spaces along the dorsal line in connection with the 



