DEVELOPMENT OF HUMAN PHARYNX 347 



as the heart descends, the epithehal tube representmg the 

 ventral diverticulum of the third pouch becomes more ventral 

 in position, the material from the pouches of the two sides 

 finally meeting upon the ventral side of the vascular funnel, 

 where cephalad of the pericardium in the anterior mediastinum 

 it becomes the thymus gland (so-called) which will be considered 

 subsequently. For illustration of the above, figures 15, 18, 

 20 and 22 may be consulted, as well as figures 1, 2, 4, 6, 7 and 8, 

 which will give the relative location of the third pouch territory 

 in succeeding stages. 



Hammar ('11) has already given an excellent description of 

 the growth and morphological differentiation of the thymus 

 and the Complex III, so that it is necessary to offer only a few 

 comments upon the transformations of the pouch as a whole, 

 in its connection with the growth shiftings accompanying the 

 descent of the heart. As is well known from the work of Hammar 

 and others, the growth of the third pouch that accompanies the 

 descent does not keep pace with it, with the results : (a) that the 

 complex as a whole moves down and the epithelial connection 

 with the remainder of the pharyngeal epithelium becomes 

 drawn into a tube or cord (ductus pharyngeo-branchialis III) and 

 broken (in embryos of about 14 mm. length). The proximal 

 (cephalic) end — the so-called thymus nodule or head, now com- 



Figs. 1-8 Were drawn by means of an Edinger projection apparatus, x 40. 

 The wall of arteries (e.g., carotid) is shown in heavy black line, pharyngeal epithe- 

 lium and surface ectoderm in solid black, the pharyngeal derivatives in oblique 

 cross line, the parathyreoid IV (fig. 8) being stippled. In some instances only 

 are muscles and nerves outlined; 'territories' are frequently indicated by inter- 

 rupted lines. 



Figs. 1, 2 and 3 Outline drawings of sections 143, 147 and 155, respectively, 

 from a 7.5 mm. human embryo (No. 256, Harvard collection). X 20. 



Figs. 4 and 5 Outline drawings of sections ^229 and ?^239, respectively, 

 from the 9.4 mm. human embryo (No. 1005, Harvard collection). X 10. 



Fig. 6 Outline drawing of section j^288, 10 mm. human embryo (No. 1000, 

 Harvard Collection). X 10. 



Fig. 7 Outline drawing of section i^228, 14.5 mm. human embryo (No. 1003, 

 Harvard Collection). X 10. 



Fig. 8 Outline drawing of section ^457, 16.4 mm. human embryo, (No. 

 1707, Harvard Collection). X 10. 



