264 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



tion, are a series of little-known bodies which have been described 

 under several names: epithelial bodies, parath5a'eoids, postbranchial 

 bodies, suprapericardial bodies, gill remnants, ultimobranchial 

 bodies, etc. These arise from parts of the visceral clefts, separate 

 from their parent tissue, and, enveloped in connective tissue, sink to 

 a deeper position. 



First of these to be mentioned are the parathyreoid bodies, which 

 apparently are the equivalents of the epithelial bodies. These arise 

 from the ventral side of the gill pouches, except in the mammals 

 where there are more dorsal in position (fig. 285, e). Of their func- 

 tions little is known except that their extirpation or disease causes 



Fig. 285. — Sections (A) of lizard and (B) of monotreme embryos, after Maurei. 

 e, third parathyreoid anlage; h, parts of heart; y, jugular vein; /, larynx; m, muscle; p, 

 pericardial cavity; th, thyreoid anlage; tm, thymus anlage; v, ventral diverticulum which 

 forms thymus in mammals; X, tenth nerve. 



effects recalling eclampsia. They are closely related in position to 

 either the thymus or the thyreoids, even coming in mammals (fig. 

 284, C), to be enveloped in the thymus gland. 



In the cyclostomes cells arise at both the dorsal and ventral sides of the gill 

 pouches which may be the homologues of the parathyreoids. None are known 

 in fishes. In the amphibia the cells are budded from two or three clefts (fig. 

 283, B); in the amniotes there are usually two contributing clefts (fig. 283, C), 

 though in some there is but one. 



Even less is known of the postbranchial or ultimobranchial bodies. These 

 arise from the last gill pocket and come to lie above the anterior end of the 

 pericardium. 



The carotid glands, occurring in amphibia and amniotes, arise at the forking 

 of the common carotid artery into its external and internal branches. Accord- 



