386 



COMPARATWE ANATOMY 



The separate mesenchymal spaces which form the anlagen of lym- 

 phatic vessels secondarily unite with one another to form the continuous 

 channels of the adult lymphatics. Later in their development, however, 

 there is good evidence that the extension of the lymphatic system involves 

 branching and sprouting of the primary lymphatic trunks. 



In the two-months human embryo paired thoracic ducts already 

 appear near and parallel to the posterior cardinal veins. They arise in 

 connexion with the paired jugular lymph sacs which lie lateral to the 

 internal jugular veins. Later the thoracic ducts become connected with 

 posterior lymph sacs and with the cistema chyli in the pelvic region. 

 From these main trunks as a basis lymphatic vessels grow into all parts 



May 19. 

 1 1 A. M. 



May 19, 

 II p. M. 



May 16, May 16, May 17. May 18, 



11.30 A. M. 11.30 P.M. II. A. M, 12.30 P. M 



Fig. 323. — Successive stages in the growth o£ a lymphatic vessel (lym.) in the tail of a 

 tadpole (Rana palustris). b.v.. Blood vessel; n., nucleus of the lymphatic vessel. 

 X 135. (From Bremer's " Text Book of Histology," after Clark.) 



of the body — head, trunk, and appendages. The jugular lymph sacs 

 acquire direct connexion with the jugular veins and retain it throughout 

 life. Although blood cells are abundant in the anlige of the lymphatic 

 vessels these cells largely disappear during ontogenesis. The lymph sacs 

 of the embryo are later replaced by lymph glands or nodes. 



During the third month of development paired lymph glands make 

 their appearance in the axillary, iliac and maxillary regions. In their 

 formation the cells of the lymph vessels or sacs do not appear to partici- 

 pate. Adenoid tissue becomes intercalated in the course of the lymphatic 

 vessel and after its envelopment by the lymphatic to form a peripheral 

 sinus, the gland becomes differentiated into an outer nodular cortex and 

 an inner mass of medullary cords. The development involves an ingrowth 

 both of lymphatic capillaries and blood capillaries, which are thus brought 

 into close relation with the adenoid, lymphocyte-forming cells. The 

 ingrowth of capillaries takes place by a process of sprouting. 



