236 LYMPHATIC SYSTEM ; SPLEEN 



which are subsequently differentiated around the nuclei to produce the lymphatic 

 endothelium. Two other views have been taken of the origin of lymphatic 

 vessels viz. (1) that of Budge (1880), which derives them from the coelom, but 

 is not supported by any satisfactory evidence, and has fallen into the background ; 

 and (2) that of Ranvier, that the lymph-vessels are derived from the veins. Miss 

 Sabin has supplied the most complete body of evidence in support of this 

 theory (that the lymphatics arise from the veins). According to her account 

 (1902-3), the lymphatic system in the pig arises from the venous endothelium at 

 four points, forming four ducts. These primary ducts dilate to form four 

 ' lymph-hearts' homologous with those of the Amphibia, though not possessed of 

 muscular walls. From these, as from centres, all the lymphatics grow first along 

 the veins towards the skin, and second along the aorta and its branches, until 

 they extend to every part of the body. The process of budding Miss Sabin 

 supposes to occur as described by Langer and Ranvier viz. solid buds are 

 formed which are afterwards hollowed out. In the formation of plexuses the 

 buds open into neighbouring ducts by absorption of the endothelium, and valves 

 are formed at the point of junction. The communication between the posterior 

 lymph-hearts and the veins is lost, but the anterior ducts persist as the right and 

 left lymphatic ducts of the adult. The primary lymph-glands she describes as 

 being formed from the four lymph-hearts. 



Lewis also derives the lymph- vessels from venous endothelium, not, however, 

 from four sites, but from several. The openings into the veins he believes to be 

 secondary, not primary, and holds that the existence of structures comparable 

 with lymph-hearts has not been demonstrated in mammals. Huntington and 

 McClure return to the idea that the lymph- vessels are mesenchymatous spaces. 

 They find that the main lymph- channels are formed along the early veins. They 

 arise as oval or spindle-shaped spaces outside the intima, in an adventitious 

 reticular tissue which takes form as the primitively redundant venous channels 

 shrink. As the intima recedes, following the diminishing column of blood, these 

 spaces increase in size and number, and, becoming confluent, form large irregular 

 channels which open secondarily into the veins. They explain the adult 

 distribution of the larger lymph- vessels by tracing them to embryonic veins, 

 which are temporary, and subsequently entirely or in great part abandoned. 



Lymph-glands. The lymph-glands arise from a plexus of lymph-vessels 

 which in section appears as a sinus broken up by connective-tissue trabeculse; 

 Capillary blood-vessels are formed in the trabeculae connected with the branches of 

 the artery of the future gland. Round the capillaries free cells (lymphocytes) gather 

 in the ..connective-tissue spaces, either introduced from the vessels (G-ulland) or 

 Droduced in situ (Saxer). The lymph-follicles (lymph-cords) are thusjlaid down, 

 and the central parts of the strands where the Ivmphocvtes are actively multiplying 

 are the aerm-centres. The original plexus of lymph-vessels forms the} sinus oi the 

 eland, and is necessarily connected with afferent and efferent channels. 1 ; 



The haemolymph-g-lands are apparently developed in a similar fashion, but 

 the jplexus is venous, not lymphatic. 



SPLEEN.- 



The spleen appears in the mesogaster as a cellular mass produced by an aggre- 

 gation ot mesenchyme-cells. It lies close to the dorsal pancreas. The cellular 

 mass becomes partially detached from the mesentery, but remains connected with 

 it by a fold (gastro-splenic omentum), through which the vessels] enter. While 



1 See further on this subject the account given in the part of this work dealing with Histology. 



2 For literature, see Hochstetter, Hertwig III. Th. ii. and iii. pp. 165-66. 





