PHARYNX AND AORTIC ARCHES 413 



is present as a small posterior thymus, made up of one or two 

 tiny lobules. Each lobule is divided into cortex and medulla, 

 the latter containing the characteristic thymic corpuscles. The 

 parathyreoid IV appears as the posterior parathyreoid gland. 

 In structure it is similar to the anterior gland, save that the 

 posterior one has the more irregular cords, larger sinusoids, and 

 a generally looser texture. The postbranchial body is repre- 

 sented, in the left aortic body, by a great group of spherical 

 vesicles surrounded by a mass of darkly nucleated cells. Most 

 of the vesicles are small and lined with low columnar epithelium. 

 A few are quite large and made of low and very regular cuboidal 

 cells. Many of the small and all of the large vesicles contain a 

 definite reddish-staining secretion. The larger vesicles resemble 

 greatly those of the thyreoid gland. The right postbranchial 

 body is represented in the adult by a few scattered clumps of 

 cells in the right aortic body. 



Thyreoid gland. The thyreoid gland is a midventral outgrowth 

 of the floor of the pharynx between the first and second pouches. 

 It is present in a 4.6-mm. embryo (fig. 2). In a 6-mm. speci- 

 men the gland is still attached to the pharynx by a slender 

 pedicle: in older embryos it is a free flattened mass placed just 

 ventral to the truncus arteriosus and the beginnings of the great 

 vessels leaving the heart. In a 9.5-mm. embryo the cells are 

 arranged in cords, some of which contain very tiny lumina. In 

 a 10. 6-mm. embryo lumina are definite; at 16.8-mm. they are 

 large and contain a reddish-staining secretion. The gland 

 retains its embryonic shape and relation to the great vessels 

 throughout Hfe. It never receives any additions from the post- 

 branchial bodies. 



General considerations. The previous work on the develop- 

 ment of the pharynx of the turtle is comprehended in the papers 

 of Van Bemmelen ('86, '88, '93), C. E. Johnson ('19), and von 

 Alten ('14, '16). Van Bemmelen finds five pouches. From the 

 first develops the tympanic cavity; the second bears a dorsal 

 epithelial bud which disappears with it. The third pouch 

 swells out into a large follicle that becomes the (anterior) thy- 

 mus and the enclosed epithelial body (parathyreoid gland). The 



