The Lymphatic System in Human Embryos. 59 



independent spindle spaces along the aygos vein, shows himself the 

 weakness of his own position. He says, referring to serial sections,^^ 

 ''These outgrowths, in the writer's estimation, constitute the veno- 

 lymphatic anlages of the thoracic and right lymphatic ducts." That 

 is to say, the entire argument rests on the interpretation of appear- 

 ances in sections, and it is becoming more clear each year that inter- 

 pretation of sections is not proof. 



The Lymphatic System in Human Embryos. 



Based upon these studies of the lymphatic system in the pig, 

 rabbit and cat embryos, I have studied through the Mall collection 

 of human embryos. I do not believe that the subject could have been 

 worked out with human embryos alone, for the real advances in the 

 study of vascular problems have always come from the method of 

 injection. The method cannot be well applied to human material 

 on account of its scarcity, but the points determined in other mamma- 

 lian embryos can be verified in serial sections of human embryos. 

 Moreover, the Mall collection is sufficiently ample to illustrate all 

 the essential points of the origin of the lymphatic system. In several 

 points I think it adds new evidence to that already gained from 

 other mammals ; for example, in connection with the history of the 

 thoracic duct, in gaining a conception of all of the primitive sacs as 

 forming a primitive system, in tracing the posterior sacs which had 

 thus far been seen only in pig embryos among mammals, and in follow- 

 ing the transformation of all the sacs into lymph nodes. It is a very 

 great pleasure to thank Professor Mall for the privilege of studying 

 his valuable collection and for many helpful suggestions during the 

 progress of the work. 



In the Mall collection no trace of a lymphatic system can be made 

 out in embryos of the first four weeks, from 2 to 8 mm. long. In 

 these there are certain spaces which might be confused with a lym- 

 phatic system, first certain areas where the meshes of the connective 

 tissue are especially large, as, for example, around the developing 

 coelom, and secondly spaces which follow the course of the nerves. 



^'McChire. Anat. Anz., XXXII Bd., 1908, p. 536. 



