410 RALPH FAUST SHANER 



The second pouch of the 9.5-mm. embryo bears a sHght dorsal 

 knob-hke outgrowth, apparently the structure described by 

 Van Bemmelen in 1893. It does not appear in younger or 

 older embryos. One is tempted to call it a transient thymus II. 

 It has, however, none of the characteristics of other thymic out- 

 growths of the same size, and must therefore be considered as 

 of doubtful nature. Both the bud and the pouch disappear in 

 a 10.6-mm. embryo, in which nothing remains of the pouch save 

 a slight furrow on the pharyngeal wall. The furrow may be 

 incorporated in the auditory tube; but if so, it has no effect on 

 the final contour of the tube. 



Third pouch. On the third pouch, in embryos of around 

 9 mm., there appear two outgrowths, a dorsal thymus III and a 

 ventral parathyreoid III. The dorsal outgrowth is preceded by 

 a slight crescentic invagination which, in a 6.6-mm. embryo, is 

 placed just medial to the branchial placode. A similac cres- 

 cent is found in snakes by Saint-Remy and Prenant. In an 

 &8-mm. embryo, the crescent is lifted up as part of the regular 

 thymic outgrowth, and there is produced a hollow, indented, 

 lobulated finger, such as is found in a 9.5-mm. embryo (fig. 8). 



The ventral outgrowth, the parathyreoid III, develops at 

 the same time. In the 9.5-mm. embryo (fig. 8), it is an un- 

 differentiated, smooth-walled, sac-like recess. 



The connections of the third pouch, first with the cervical 

 sinus and then with the pharynx proper, become now broken, 

 so that in a 10.6-mm. embryo (fig. 9) the thymic and para- 

 thyreoid outgrowths, with the intermediate pouch-tissue, form 

 an independent complex which lies free in the surrounding 

 mesenchyma, just behind the internal carotid artery. 



The dorsal thymic outgrowth grows very rapidly. Its central 

 cavity disappears; the intermediate pouch-tissue is absorbed; 

 and the parathyreoid gland is completely surrounded by the 

 downward extending lobules of the thymus. In a 16.8-mm. 

 embryo (Harvard Embryological Collection, 1090) the cells 

 of the thymus take on the lymphoid appearance typical of 

 thymic tissue. In a 32-mm. embryo (H. E. C. 1127) the lobules 

 show subdivision into cortex and medulla, and within the latter 

 appear thj^mic corpuscles. 



