nasal pouch 

 Rathke's pouch 



stomodeum 

 pituitary 



B 



otic vesicle 



notochord 



dorsal aorta 



notochord 



nasal pouch 



Rathke's pouch 



nasal pouch 



■^secondary pharyngeal membrane 

 opens to outside later 



Rathke's pouch opened 

 into pharynx 



Figure 12-1. Stages in the development of Rathke's pouch in the lamprey and Eptofrefus. (After 

 Dohrn, and Kupper, 1900) 



of the protochordate as in Amphioxus. In Amphioxus the 

 bilobed nature of the endostyle when it first appears sug- 

 gests a pair of pharyngeal pouches (Van Wijhe). 



The endostyle evaginates early in the lamprey and soon 

 differentiates into a complex and apparently mucoid sec- 

 retory organ. Some of the cells of this organ take up iodine very 

 much in the way of the thyroid tissue, which develops later. 

 With the onset of metamorphosis, the endostylar gland 

 loses connection with the pharynx and a part of its tissue 

 forms thyroid-like follicles. In the hagfish the thyroid arises 

 as a groove extending the length of the floor of the pharynx. 



In the actinopterygian fishes the thyroid arises, along a 

 line, from the floor of the pharynx. From this groove, clumps 

 of cells come to lie along the course of the ventral aorta. Each 

 clump forms a follicle or groups of follicles of a typical thy- 

 roid structure. Experimental evidence indicates that their 

 secretion is of a thyroid nature. A few teleosts have a mas- 

 sive thyroid like that of a shark. 



In the sharks the thyroid tissue is rather gelatinous, but 

 formed of typical follicles, and encapsulated by connective 

 tissue. This mass lies far forward, behind or below the basi- 

 hyal cartilage, anterior to the forking of the ventral aorta. It 

 may be crescentic in shape or irregular. 



In the shark the gland arises as an evagination from the 

 floor of the pharyngeal cavity with or without a central 

 lumen. As it sinks deeper it loses connection with the phar- 



ynx. In Chlamydoselachus the duct retains its connection with 

 the pharynx even in the adult. This duct enters the pharynx 

 through a perforation in the basihyal cartilage. In the lining 

 of the duct, numerous scale-like structures are present, per- 

 haps remnants of stomodeal denticles. 



In amphibians the gland arises as a midline element but 

 divides into two parts which lie close below the basihyal in 

 the frog or well out to either side in the salamander. Lati- 

 meria, the coelacanth, has a compact midline thyroid lying 



THE THYMUS, PARATHYROID, AND 

 ULTIMOBRANCHIAL BODY 



In the mammal the thymus arises from evaginations from 

 the dorsal wall of pharyngeal pouches III and IV, while 

 the parathyroid arises from anterior outpocketings from 

 these same pouches (Figure 12-2). From the fifth pouch a 

 pair of evaginations produce the ultimobranchial body. In 

 man the thymus evaginations of the third arch form the de- 

 finitive gland. The evagination of the fourth arch remains 

 rudimentary and becomes embedded in the thyroid. In 

 some mammals these elements may remain as distinct thy- 

 mus masses. During development this gland is moved back 

 into the upper chest or base of the neck as a result of the 

 constricted nature of the neck. 



380 



THE ENDOCRINE GLANDS 



