182 THE FEOG. 



the carotid gland is formed, ai-e developed, at a slightly later 

 stage, between the ventral ends of the afferent and efferent 

 vessels of the second and third branchial arches, and perhaps 

 in the fourth arch as well ; and it is by perforation of these 

 plates that the direct communications between the afferent and 

 efferent vessels are established. After the metamorphosis the 

 epithelial plates persist, and even increase in size, forming solid 

 epithelial bodies, lying in the floor of the mouth, in the angles, 

 between the carotid and systemic, and systemic and pulmo- 

 cutaneous arches respectively. They acquire connective tissue 

 capsules, and have been spoken of as accessoi'y thyroid bodies. 



c. . The hinder part of the dorsal aor ta. The union of the two 

 aortse to form the definite dorsal aorta occurs almost immediately 

 behind the pharyngeal region (Fig. 77). 



At, or close to, their point of union each aorta has, at the 

 time of hatching, a small bulging of its ventral wall. Later on, 

 these bulgings increase in size and become sacculated, forming 

 a pair of prominent, pigmented, thick-walled swellings, th^^lo- 

 jiernli of the head kidnevs (Figs. 77 and 78, gm), which hang 

 down into the dorsal part of the body cavity, lying opposite the 

 head kidneys, kp, along almost their entire length. 



In their further development, the glomeruli keep pace with 

 the head kidneys ; they remain large up to about 23 mm., i.e. so 

 long as the head kidneys remain functional (c/. Figs. 83, 84, and 

 85) ; but in the later stages, when the head kidneys begin to 

 degenerate, the glomeruli also become smaller. They are still 

 present, though of very small size, up to the end of the first 

 year, but disappear completely in the second year. Certain 

 further points in connection with the glomeruli will be noted 

 in the section dealing with the kidneys. 



d. The pulmonary arteries arise, as described above, shortly 

 after the time of hatching (Figs. 77, 78, AP), as diverticula from 

 the dorsal ends of the efferent vessels of the fourth branchial 

 arches. As the connection of the afferent vessel of this arch 

 with the truncus arteriosus is not acquired until some time after 

 the mouth opening is established, there is a considerable period 

 during which the supply of blood to the lungs is derived ft'om 

 the aorta and not from the heart, i.e. is arterial and not venous; 

 a condition which suggests that the lungs had originally some 

 function other than respiration to fulfil. 



