DEVELOPMENT OF THE LYMPHATICS OF THE LUNGS L\ THE EMBRYO PIG. 49 



in the literature regarding the development of the cardiac and pulmonary lymphatics 

 from the right duct, or the morphological fate of the right duct in mammals. In 

 the same report attention was called to the fact that the lung- vessels can be injected 

 from the retroperitoneal sac, but this was not studied further at that time. The 

 right duct grows primarily to the heart, just as the left grows to the aorta, this 

 asymmetry depending upon that of the cardio-vascular system, according to the 

 general rule that the principal lymphatic trunks follow the large blood-vessels, and 

 grow with the greatest rapidity where the blood-supply is most abundant. 



In the beginning I wish to lay emphasis upon the fact that the lung lymphatics 

 develop partly from the retroperitoneal sac, and to call attention to the fact that 

 these vessels persist in the adult as part of the permanent drainage of the lung, and 

 hence may be of importance in patholog3^ On account of the comjilexity of the 

 development of the lung lymphatics, it has seemed Ijest to i)resent this work, not 

 by describing and figuring a series of progressively more complex specimens, but 

 by describing the development as a consecutive growth and illustrating with those 

 preparations that may seem best to clarify the text. However, as a matter of ref- 

 erence, the following table of periods has been arranged, in order to offer a brief 

 outline of the complexity at varying stages. These stages are selected with regard 

 to the more important principles of growth and are as follows: 



(1) The downgrowth of the two ducts, completion of the primitive system, and the first vessels 

 to the trachea and lungs. Embryos 2.3 to 3.5 cm. 



(2) The migration of the heart; the coalescence of the cardiac drainage with that of the lungs, by 

 the formation of the tracheal plexus and the plexus on the arch of the aorta; the growth of the vessels 

 in tlic lung from the earliest sprouts along the bronchi to the primitive pleural plexus, and the early 

 marking-off of the connective-tissue septa; and the growth up from the retroperitoneal sac through the 

 ligamentum latum and the anastomosis in the primitive septa into which the vessels grow. Embryos 

 3.5 to 4.5 cm. 



(3) The completion of the primary lymphatic system; that is, when the entire organ is supplied , 

 and the further development is in an increasing complexity of the plexuses already present, incident 

 to the increase in the size of the organ and its assumption of mature activities. During this period 

 the formation of the valves and nodes begins. Embryos 4.5 to 7 cm. 



(4) The remainder of the development is considered a period, as it is, in reality, an adaptation of 

 the system already present to the increasing needs of the organ. This includes the differentiation of 

 the drainage-lines and the final development of the nodes. 



In describing the development of the lymphatics of the lung, the growth of the 

 left duct down to the aorta, of the right duct to the heart, and the formation of the 

 primitive tracheal plexus and the early vessels to the lungs from both ducts will be 

 considered first; the further development of the tracheal plexus, together with the 

 changes incident to the descent of the heart, will follow; then the origin of the 

 vessels from the retroperitoneal sac and their growth up through the ligamentum 

 pulmonale into the lungs will be considered. After the anastomoses of the two 

 sets of lymphatics, the lung will be considered as a whole, inasmuch as the further 

 development is symmetrical for the entire organ, with the exception of the final lines 

 of drainage and the development of the nodes. 



I wish to express here my indebtedness to Professor F. R. Sabin for her con- 

 stant advice and assistance throughout this work. Also I wish to thank Mr. James 

 F. Didusch and Miss Flora Schaeffer for the illustrations. 



