288 HISTORY OF THE HUMAN BODY 



only the last three possess definite thymus-anlagen, while those 

 of the first two are transitory rudiments. In the lizard, with 

 four embryonic pharyngeal pockets, the first has a rudimentary 

 thymus-anlage, and the second and third functional ones. 

 The fourth is without one. In mammals the definite thymus 

 is formed either wholly from the anlage associated with the 

 third pocket, or else mainly from this with the addition of a 

 small contribution from the fourth. 



The fate of the ventral anlagen, that form the epithelial 

 corpuscles, is known only in part, but they form at best only 

 small nodules, of a more or less glandular nature, and never 

 attain the bulk of a thymus. In amphibians that of the second 

 pocket becomes associated with the carotid artery and forms 

 the so-called " carotid gland," especially conspicuous in frogs 

 and toads. Those of the third and first pockets are developed 

 as epithelial corpuscles. In the lizard the only ventral an- 

 lage that persists is that of the third pocket, which forms a 

 carotid gland. In Echidna epithelial bodies appear, associated 

 with pockets two and four, that of the second becoming a 

 carotid gland as in amphibians. In other mammals pockets 

 three and four, or pocket four alone, are provided with such 

 anlagen. These may remain associated with their respective 

 thymus anlagen, or may become detached, but possess little if 

 any physiological significance. 



A third system of organs associated with this region ap- 

 pears typically as a single pair of evaginations, which arise 

 from the floor of the pharynx, in all cases immediately behind 

 the last gill-slit. From their secondary relation to the peri- 

 cardium in selachians they have received the name of siipra- 

 pencardial bodies, but they are better termed the post-bran- 

 chial bodies, a name which expresses a fundamental and uni- 

 versal relationship. In selachians both members of the single 

 pair of these organs develop, but in urodeles and in lizards 

 the left alone completes its development, while the right one 

 remains in a rudimentary condition, and eventually disap- 

 pears. The occurrence of post-branchial bodies in birds and 

 mammals is uncertain, but some identify with these a pair of 



