CHAPTER XIII 



THE FUNCTIONS OF THE THYROID AND PARATHYROIDS 



A. Introductory : The Organs derived from the 

 Region of the Pharynx and Gill -Clefts 



THE organs in question may be divided into two groups. The 

 first group is represented by structures which arise simul- 

 taneously with the gills and gill-clefts viz., the thyroid, the 

 thymus, and the post-branchial bodies (the suprapericardial 

 bodies of v. Bemmelen). The organs of the second group are 

 developed concomitantly with the degeneration of the gills and 

 the closing of the clefts, from whose epithelium they arise. 

 These are the " branchial bodies " of the Anura and the para- 

 thyroids throughout Amniota. 



B. The Comparative Anatomy and Histology of the 



Thyroid and Parathyroid Bodies 



Cephalochorda and Cyclostomata 



1 . In Amphioxus and Ammoccetes the thyroid is a sausage- 

 shaped glandular body which branches in a fork-like manner 

 and retains its opening into the pharynx. This indicates a 

 relationship to the hypobranchial furrow or endostyle of 

 Tunicates. The epithelium lining this " saccular gland is 

 ciliated, and secretes mucus. 



In Petromyzon the connection with the pharynx is broken, 

 and the gland consists of a number of closed vesicles lined by 

 tall columnar epithelium, and containing a colloid substance 

 in their interior. 



The details of the metamorphosis of the endostyle in 

 Petromyzon have been recently worked out by Marine. The 

 endostyle persists throughout the larval period in full activity. 

 Then, at the metamorphosis, it undergoes involution, all but a 

 remnant disappearing. This remnant assumes the form of the 



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