ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 527 



ring in the course of the larger lymph-channels. The first pair are 

 known as the cervicothoracic nodules. Each represents a widening of 

 the lymphatic vessel which runs beside the jugular vein and is closely 

 apposed to the terminal course of the jugular. The second pair lies 

 close under the vertebral column at the level of the reproductive organs ; 

 they are formed from the lymphatic vessels which run along each side 

 of the aorta, and are known as the lumbar nodules. 



The lymphatic system arises by budding from the veuous system at 

 the two points where a permanent communication between the two 

 systems exists — that is from the superior venae cavre and from the pelvic 

 veins. The lymphatic nodules begin to develop about the fifteenth day. 

 A thickened zone of mesenchyme is laid down round the previously 

 simple tube-shaped lymphatic vessel, and strands of mesenchyme grow 

 forward into the lumen so that it is transformed into a reticulated 

 system of smaller lymph-spaces Up to this point the development has 

 been analogous to that of the nodules in Mammals, but it now diverges, 

 for instead of a peripheral sinus, a " main lymphatic space " is formed, 

 .which runs a central course as a direct continuation of the primitive 

 lymphatic vessel, from one pole of the rudiment to the other. It is 

 everywhere connected with the smaller peripheral spaces. The infiltra- 

 tion of leucocytes takes place round the main space, and gives rise to 

 central lymph follicles. 



Thus the arrangement of elements in a typical lymphatic nodule in 

 Birds is the opposite of that in Mammals, and a cortical and medullary 

 substance cannot be distinguished. As the follicle grows towards the 

 " main lymph space " the latter becomes narrow and irregular, so that in 

 the later stages it is difficult to recognize. Towards the end of the first 

 month after hatching lymphoblasts' appear in the follicles and there is a 

 subsequent appearance of germinative centres ; towards the end of the 

 second month the development of the lymphatic nodules is complete. 

 No capsule is developed in the lymphatic nodules of bird i, and therefore 

 all the structures arising from the capsule in Mammals are also absent. 

 The delimiting portion of the bird's lymphatic nodule is itself composed 

 of glandular substance, and is not divided up by partition walls ; the 

 lymphatic tracts are entirely free. The main substance may be divided 

 into the walls, the lymphatic strands, and the lymphatic follicles. The 

 first two are exactly alike and of very simple structure ; they consist of 

 fibrillar connective tissue whose wide meshes are quite full of leucocytes. 

 The lymphatic follicles are simple extensions of the strands in which 

 the meshes have been pushed farther apart by the more abundant in- 

 filtration of leucocytes. The walls, lymphatic strands, and follicles are 

 divided off from the lymph spaces by a fine layer of endothelium. As 

 in Mammals, the germinative centres form one of the specific constituents 

 of the lymphatic follicles, and they are definitely bounded by circular 

 connective tissue fibres, and surrounded by a fine network of capil- 

 laries. They are more constant structures in the lymphatic nodules of 

 Birds than in those of Mammals. There is no hilum in the lymphatic 

 nodules of birds ; blood-vessels and lymph-vessels may enter them at 

 different points. 



