The Origin and Development of the Lymphatic System. 27 



shows in fig. 437 (Evans 35). Each of the five spinal veins drains 

 a plexus of wide capillaries, which marks the presence of the develop- 

 ing spinal ganglia. 



It is, however, the superficial veins which are of more interest from 

 the standpoint of the lymphatic system. Along the dorso-lateral bor- 

 der of the cardinal veins there is a series of intersegmental sprouts, 

 varying from one to four or five in each interspace. These sprouts 

 drain a superficial capillary plexus which is developing in a groove 

 in the neck which is a continuation of the groove dorsal to the Wolffian 

 ridge. The arteries for this superficial capillary plexus come from the 

 deep segmental arteries (Evans 35, fig. 437). The connections of 

 the plexus with the cardinal vein are most abundant near the poste- 

 rior cerebral vein. This superficial capillary plexus of the body wall 

 has a most interesting development. For the area between the limb 

 buds it has been worked out by Dr. Helen W. Smith (141), whose 

 figures are copied by Evans (35). This superficial plexus in the tail 

 region shows a constant zone of widened capillaries (35, fig. 467) 

 corresponding to the position of the posterior lymph hearts in birds. 

 For the neck region it is necessary to know this plexus in order to 

 locate the jugular lymphatics, and therefore its development will be 

 followed in further stages. 



The ventral branches of the anterior cardinal vein are represented 

 at this stage by one large pericardinal vein, which receives branches 

 from the gill arch region. The common stem for the branchial and 

 pericardial vessels connects with the anterior cardinal vein, but it 

 drains directly into the duct of Cuvier (see Grosser and F. T. Lewis 

 as quoted by Evans 35, p. 660) on the right side of this embryo, 

 while on the left side of the same specimen it opens into the anterior 

 cardinal vein. 



The earliest lymphatic buds have been found in embryo pigs 

 measuring from 10 to 11 mm. long. All of my measurements mean 

 the greatest length and are made on the fresh specimen. The stage 

 of the first lymphatics is shown in two figures, fig. 5, a total mount 

 of an embryo injected with India ink through the umbilical artery 

 in a specimen 11 mm. long, and fig. 6, a cross section 250 ^ thick, 

 through the lymphatic area in an embryo measuring 11 mm. The 

 blood vessels were injected with India ink, and the lymphatics are filled 

 with blood. 



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