PLATE 2 



EXPLANATION OF FIGURES 



27, 28 Transverse sections showing the relative positions of the fourth and 

 fifth thymus bodies (/iv) (^v) of a 19 mm. larva. (X 50) 



29 Transverse section through the central region of the third thymus body 

 of 25 mm. larva, showing the comparatively few large epithelial cells forming the 

 margin of the body, within which is a conspicuous space. Note there is as yet no 

 connective tissue envelop about the margin, the cells are simply imbedded in 

 loose connective tissue cells. (X 200) 



30 Transverse section through the central portion of the third thymus body 

 of a 35 mm. larva, showing the now scattered smaller epithelial cells surrounded 

 by a connective tissue envelop. (X 200) 



31 Transverse section through the central region of the thyreoid gland of 

 a 45 mm. larva, showing the arrangement of the thyreoid follicles (fol.) to one 

 another, and to the vascular and lymph vessels (v.v.). Note that the inferior 

 jugular vein (e.j.) is almost surrounded by the follicles, and is enclosed like them 

 with the 'membrana propria' of the connective tissue, while the external carotid 

 artery (e.c.) is not thus enveloped. (X 200) 



32, 33, 34 Transverse sections through the third, fourth and fifth thymus 

 bodies (<ni) (hv) (tv) of an early transforming larva. Note the condition of the 

 now transforming pharyngeal pouches. (X 15) 



35. Transverse section through the head of a young adult, showing the posi- 

 tion of the second epithelial body (e.b.o), and the thymus gland (t). (X 15) 



36 Schematic ventral view of the head of a young adult, showing the relative 

 positions of the thyreoid {ir.), thymus (t) glands and epithelial bodies (e.b.), to 

 the blood vessels and the muscles of the region. (X 4) 



37 Lateral view of adult showing the position of the three-lobed thymus 

 gland. (X 4) 



38 Wax reconstruction showing the relationships of the postbranchial body 

 (p6.) of a 19 mm. larva. (X 50) 



39 Transverse section showing the early anlagen of the epithelial bodies 

 (e.b.1,2). (X 25) 



40 Schematic lateral view of the head of adult, showing the position of the 

 thymus (t) and thyreoid (tr.) glands, epithelial bodies (e.b.) and the blood vessels. 

 (X4) 



41 Wax reconstruction of the three thymus bodies of a 19 mm. larva (dorsal 

 view of right side), showing their relative position to the branchial cartilages, 

 (1, 2, 3, 4) blood vessels, and nerves of the region (711,2,3). (X 50) 



42 Wax reconstruction of the region of the epithelial bodies (e.b.) of a young 

 adult on the left side, showing their relation to the blood vessels, carotid glands, 

 thymus and thyreoid glands, etc. (X 50) (See also fig. 35, in section.) 



(Expkmation of figtires 43 to 47 on page 680) 



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