327 



place result only from the adaptation to the changes which occur in 

 the parts through which it passes. 



Di.ssections. — Successful dissections of the primary Subclavian, 

 and of the earlier stages of the secondary vessel, were very difficult 



--n. ch. 



' L... ^--n. ch. 



d. ao,- 

 al. c 



. — -c. V. 



..^a. 5. 



A. 4. 's:—- 



■r-—<r-~~-^^^-a. 3. 



d. ao 



al. c.^ 



a. 4. 



set. a. s. 



18. 



19. 



).h. 



sd. a. p. 



scl. a. 5./ 



w. b. 



20. \'- \ 



Figs. 15, 16, and 17, sections through the wing region of a chick embryo of six 

 days of incubation, when the Subclavians of both sides with their segmental branches 

 arise from the Aorta by a single, median, dorsal branch. In Figs. 16 and 17 the 

 juncture of the primary and secondary Subclavians is shown. The sections, Nos. 5, 6, 

 and 10 on the slide, represent a thickness of 80 [A (x about 25). Reference letters as 

 in proceeding figui-es. 



Figs. 18 and 19, sections through the region of the Aortic arches of the same 

 embiyo as figs. 15 — 17, showing the anterior portion of the secondary Subclavian, 

 springing from the third arch. The sections are 5 and 7 on the slide, and represent 

 a thickness of 40 /< (x about 22). Fig. 20, diagram of a section through the same 

 region as figs. 15 — 17, showing the form of the primary Subclavian at this age, and 

 the point of junction of the primary with the secondary vessel. 



a. 3., a. If., and a. 5., third, fourth and fifth Aortic arches. Other reference letters 

 as in preceediug figures. 



