372 The Development of the Lymphatic System 



Cffilom, wliich could form a thoracic duct as Budge supposed. In this 

 stage, before there are any lymphatics, many of the blood-vessels widen 

 out into sinusoids instead of capillaries.' Some of these sinusoids are 

 beneath the skin, and since they are many times the width of the 

 capillaries, and since the endotheliimi which lines them is thinner than 

 that of the capillaries, they look much like lymphatics. However, they 

 are readily distinguished by their evident connection with the veins 

 and by the fact that they contain blood. 



The development of the lymphatic system was found in this way. 

 We have an abundant supply of pig embryos at the Anatomical Labor- 

 atory. Every day large numbers of embryos of all sizes from under 

 10 mm. upwards are brought to the laboratory. Moreover, we are so 

 near tlie abattoir that the embryos are often brought with the heart 

 still beating. It is essential in injecting lymphatics to have fresh 

 embryos, for after an embryo is once thoroughly cold it is impossible to 

 get good injections. The best results are always obtained while the 

 heart is still beating. The embryos must be injected immediately 

 after removing them from the nterus and the skin must be kept moist 

 while injecting. «?» 



I began with the study of the lymph glands and made the first 

 injections of them by introducing the needle into the foot pads. If 

 in pigs about 10 cm. long, the needle is inserted into the foot pads of 

 the hind feet, ducts are readily injected which run to a gland in the 

 inguinal region, while from the fore feet the ducts run up to a gland 

 in the front of the neck. As younger pigs were taken, for example, 

 below C cm., it became impossible to inject any ducts from the foot 

 pads; and still younger, at 4 cm., it became impossible to inject the 

 ducts subcutaneously in the side of the leg. In order to get younger 

 stages of the glands it was thus necessary to inject nearer to them and 

 it was found that in stages when no lymphatics could be injected in 

 the legs they could still be injected with ease in the body wall. These 

 ducts in the body wall run to two other glands, one over the crest of 

 the ileum and one in the po^erior part of the neck. 



These injections gave the first idea of the gradual growth of the 

 lymphatic system from the centre; because at a certain stage when 

 jymph ducts could always be injected in the body wall, none were ever 

 injected in the feet or legs; that is to say, the legs had not yet received 



'Minot: On a hitherto unrecognized form of blood circulation without capillaries 

 in the organs of vertebrata. Proceedings of the Boston Society of Natural History, 

 Vol. XXIX, No. 10, April, 1900, pp. 18.5-31.5. 



