412 RALPH FAUST SHANER 



postbrancliial bod}^, form another independent complex, placed 

 just behind the systemic aortic arch. 



The dorsal thymic outgrowth, the thymus IV, undergoes a 

 transformation similar to that of thymus III, save that the 

 thymus of the fourth pouch is always very small and never 

 envelops the parathyreoid gland. The thymus IV apparently 

 absorbs the intermediate pouch-tissue. 



The ventral outgrowth, the parathyreoid IV, can be followed 

 in older embryos as a large organ, as large as the corresponding 

 outgrowth of the third pouch. The changes in structure of both 

 are quite similar. 



Fifth pouch. The transient fifth pouch, at the height of its 

 development, displays a slight crescentic invagination (8.4 mm., 

 H. E. C. 1078), which can be interpreted as a rudimentary thymus 

 V. The whole pouch, thymus and all, then degenerates. It 

 may be represented in the 9.5-mm. embryo (fig. 8) by a heap of 

 cells placed at the point where the postbranchial body joins the 

 fourth pouch. No later traces of the pouch or thymus can 

 be found. 



Postbranchial body. The postbranchial body at first develops 

 equally well on both sides of the embryo. In the 9.5-mm. em- 

 bryo (fig. 8), the organ is a large hollow flask wdth a very narrow 

 neck. When the fourth pouch loses connection with the cervical 

 sinus and the pharynx proper, the postbranchial body is carried 

 away with it to form a part of the complex already referred to 

 (fig. 10). The right postbranchial body now degenerates. 

 Traces of it can be found in older embryos and in the adult. 

 The left body increases rapidly in size and becomes studded with 

 epithehal buds (fig. 10). In older embryos the buds grow at 

 the expense of the original sac, which in a 32-mm. embryo (H. E. 

 C. 1127) is reduced to a group of isolated epithelial vesicles. 



The thymus IV, the parathyreoid IV, and the postbranchial 

 body develop throughout embryonic life as a closely associated 

 group of organs. All three can be found in the adult as com- 

 ponents of the 'aortic body' of Van Bemmelen ('88), situated 

 along the systemic aortic arch just above the ductus arteriosus. 

 A section of this body is shown in figure 15. The thymus IV 



