588 



THE EMBRYOLOGY OF THE FROG 



The Carotid Glands form from the ventral ends of the anterior gill 

 pouches as epithelial proliferations at about the time the internal gills 

 appear. 



The Pseudothyroid Body is a small outgrowth in the postero-ventral 

 branchial region, apparently having no relation with the disappearing 

 gill-clefts. This body disappears with the exception of traces of the mid- 

 dle and ventral portions, which persist for a short time after metamor- 

 phosis, but then they, too, disappear. 



The Ultimobranchial Bodies 

 (also called post-branchial or supra- 

 pericardial bodies) lie posterior to 

 the fifth visceral pouch. They have 

 formed as solid proliferations from 

 the pharyngeal wall just behind the 

 visceral (fifth) pouch, (fourth 

 branchial pouch). These bodies are 

 supposed to represent vestigial por- 

 tions of a sixth pair of visceral 

 pouches, although they do not ex- 

 tend to the surface ectoderm. They 

 separate from the pharynx and 

 acquire a lumen, coming to lie on 

 the floor of the pharynx in a supra- 

 pericardial position. (Fig. 342.) 



The Thyroid Body is formed as 

 a medial invagination from the floor of the pharynx just a short time 

 before hatching. It forms as a solid rod of cells, but a few days after 

 the opening of the mouth it forms a pair of bodies which grows very 

 rapidly and becomes very vascular. The thyroid body has no genetic 

 relationship to the branchial, structures nor do any of the following 

 structures possess such relationship. 



The lungs. These develop just before hatching as a pair of solid 

 proliferations from the ventral wall of the posterior portion of the fore- 

 gut, just between the yolk-mass and the heart. The cavities begin to 

 form in the proximal region. 



The Laryngeal Chamber (Fig. 332, B) is formed by the wall of the 

 fore-gut between and around the lung diverticula which become de- 

 pressed and form a groove, which then (at least partially) constricts off 

 from the alimentary tract. 



The Glottis is the opening which remains in the laryngeal chamber 

 as it constricts. 



The Tongue appears just before metamorphosis as an elevation in 

 the floor of the anterior portion of the pharynx, just behind the thyroid 

 region. There is a glandular depression directly in front of the elevation 



Fig. 343. 



From a model of the duodenum and the 

 primary evaginations of the liver and pan- 

 creas in a 5 mm. sheep embryo. D.pan., Dor- 

 sal pancreas ; Du., duodenum ; D.ch., ductus 

 choledochus ; G.bl., gall bladder ; H.du., hepatic 

 duct. (After Stoss.) 



