PHARYNX AND AORTIC ARCHES 411 



The ventral part of the complex, the parathyreoid III, in- 

 creases in size much more slowly. Its internal cavity can 

 still be found in a 16.8-mm. embryo, in which it is lined by 

 cuboidal epitheUum. The substance of the parathyreoid gland 

 is then invaded by vascular and connective tissue, and the en- 

 todermal cells are rearranged into solid cords. In a 27mm. 

 embryo (H. E. C. 1096), the central cavity is lost, and the para- 

 thyreoid gland is like the adult organ in all respects. 



The two derivatives of the third pouch are present regularly 

 in the adult Chrysemys picta as a large anterior thymus (tig. 

 14), placed at the union of the internal carotid and the sub- 

 clavian arteries, and, within it, the anterior parathyreoid. The 

 thymus is divided into many lobules bound together by dense 

 connective tissue. Each lobule is easily divisible into a dense 

 cortex and loose medulla. Within the medullary substance 

 occur large irregular thymic corpuscles. Both the medulla and 

 the corpuscles are tinged red with large eosinophilic cells, the 

 protoplasm of which is filled with coarse and brilliant staining 

 granules. 



The anterior parathyreoid gland is a small round body made 

 up of solid cords of irregular epithelial cells. The cords are 

 surrounded by a rich network of sinusoids and only a small 

 amount of connective tissue. 



Fourth pouch. Two outgrowths arise also from the fourth 

 pouch. The small dorsal thymus is preceded by a slight cres- 

 centic invagination, less well-defined than in the third pouch, 

 but nevertheless identifiable in an 8.8-mm. embryo (H. E. C. 

 1086). The crescent is lost at once, and a small solid outgrowth 

 is formed, such as appears in the 9.5-mm. embryo (fig. 8). The 

 ventral diverticulum, the parathyreoid IV, is even larger in the 

 9.5-mm. embryo than the corresponding outgrowth of the third 

 pouch, with which it is identical in structure. 



The stalk connecting the fourth pouch with the cervical 

 sinus, and the common neck by which the fourth pouch and the 

 postbranchial body are joined to the pharynx proper, are next 

 destroyed, so that in a 10.6-mm. embryo (fig. 10) the two out- 

 growths, with the intermediate pouch-tissue and the attached 



