70 Florence B. Sabin. 



best stage for studying the formation of the sinuses. The question 

 of the development of reticulum has been taken up by Mall (84). 



In regard to the formation of the lymph cords and follicles, the funda- 

 mental point is the question of the origin of the lymphocytes themselves. 

 This, however, I shall not undertake to discuss. The lymphocytes may 

 occur diffusely in all of the trabecule of the developing node and this 

 is the simplest form of a gland. This form is shown by Jolly (59) 

 in his plate IX, iig. 3, for the lumbar glands in birds, by Kling (72), 

 plate XXVII, fig. 11, for the axillary glands in human embryos and 

 in fig. 9 (Sabin 131) from the primary jugular lymph glands in 

 the pig. It will be noted that all of these figures are from primary 

 lymph glands. The definite follicles may develop in the center or in 

 the periphery of a node, and there is every possible variation in 

 the proportion of the diffuse cords and the definite follicles. There 

 are two processes in the development of the follicle: (1) an increase 

 in the number of lymphocytes forming a definite clump, and (2) 

 the formation of a tuft of blood capillaries. Both the cords and the 

 follicles form along the blood vessels, the follicles coming at the cap- 

 illary bed. 



As I have said, the early glands which come from the primary sacs 

 pass through the form of diffuse distribution of the lymphocytes. 

 Follicles which develop later, however, may begin at once as clumps 

 of lymphocytes, making very definite follicles surrounded by a lym- 

 phatic plexus. A number of such tiny follicles are to be seen in fig. 

 16 (Sabin 131), which is the developing group of inguinal lymph 

 glands in a pig 24.5 em. long. 



Lymph glands may be either simple, consisting of one follicle with 

 a peripheral sinus, or compound, with many follicles and cords, and 

 both peripheral and central sinuses. 



The subject of hsemolymph glands has been taken up by Helly in 

 the Brgebnisse fur Anatomie and Entwickelungsgeschiehte for 1902. 



VII. COMPARATIVE MORPHOLOGY OP THE PRIMARY 

 LYMPH SACS, LYMPH HEARTS, AMPHIBIAN LYMPH 

 SACS AND LYMPH GLANDS. 



We are now in a position to consider the comparative morphology 

 of lymph sacs and lymph hearts. I shall not attempt to analyze the 

 work of Favaro (36a) and of Allen (2-4) on lymphatics in fishes, 



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