388 The Development of the Lymphatic System 



certain tissues which they never reach even in the adult, for example, 

 the cornea and cartilage. 



The development of the lymphatics has here been traced for the 

 most part in pig embryos. We have, however, a rat embryo which 

 corresponds with Fig. 6. The veins are filled with blood and so the 

 lymph hearts are easy to find. Saxer ^ evidently has a cow's embryo 

 2.5 cm. long cut in serial sections, which shows the anterior lymph 

 hearts. He describes the specimen as follows, that in the side of the 

 neck are small lymphatic ducts which collect into two symmetrical 

 cystic spaces, from which a duct narrows down rapidly and opens into 

 the vein. Several of the human embryos of Professor Mall's collec- 

 tion have the veins well filled with blood, so that it has been possible 

 to find the lymphatics in them and to confirm some of the steps of the 

 development of the lymphatic system in the human embryo. 



The lymphatic system, at the stage to which it has been traced in 

 this work, represents about the stage of the adult frog. That is, it 

 is a system of duets without valves except at the openings into the 

 veins and generally speaking, without glands. The plexuses which 

 are to form the glands are present in many places. 



The function of such a system is clearly suggested by a pathological 

 specimen reported by Smith and Birmingham." The specimen was 

 of twins prematurely born, one of which was normal, while the other 

 was so oedematous that it was simply a round ball. By good fortune 

 the writers studied the lymphatic system in both foetuses. In the 

 normal one the thoracic duct and lymph glands were easily found, while 

 in the oedematous one there was no trace of a thoracic duct and no 

 lymph glands. 



The lymphatic system is a modification of the circulatory system, 

 dependent both in its origin and, in large measure, in its development 

 on the blood-vessels. It returns to the vascular system the fluid 

 exuded into the tissue spaces from the blood-vessels. Speaking more 

 generally, it is a system of absorbents. The lymph glands which de- 

 velop by the increase of connective tissue around plexuses of ducts 

 come later; they occur only in birds and mammals and do not begin 

 to develop in mammalian embryos until the ducts or capillaries they 

 drain are well formed. 



This study was begun at the suggestion of Professor Franklin P. 



9 Saxer: Anatomische Hefte 1, VI, 1896, S. 370. 



'0 Smith and Birmingham : Absent thoracic duct causing redema of a foetus. Jour- 

 nal of Anatomy and Physiology, Vol. 23, 1889, p. 532. 



