THYMUS 



169 



entirely separated from the entodermic surface from which they have taken origin, 

 and are converted into cell- cylinders in which colloid is ultimately formed. 



The lateral rudiment of the thyroid apparently corresponds to a structure present in most 

 vertebrates, and known as the post-branchial body. It is an entodermic pocket which gives 

 origin to an epithelial body, but this does not yield colloid except in mammals. In Echidna 

 (Maurer) there is colloid formation, but the body remains distinct from the primitive median 

 thyroid. Only in placental mammals does it become united with the median rudiment. The 

 account given above is founded on Bern's original description, but it is right to say that some 







<,*. 







rues. thyr. 



CM. thym 



FIG. 212. TRANSVERSE SECTION THROUGH THE NECK or A HUMAN EMBRYO OF 15'5 MM., TO SHOW THE 



POSITION OF THE THYROID AND THYMUS GLANDS, AND HISTOGENESIS OF WALLS OF O3SOPHAGUS 



AND TRACHEA. (T. H. Bryce.) 



not, notochord in vertebral body ; sy, sympathetic ganglion ; vg, vagus ; j.v., jugular vein ; 

 c.a., carotid artery ; thym, thymus ; mes. thyr., mesial thyroid. 



hold that, while the diverticulum from the fourth pouch becomes included as a vesicle in the 

 lateral lobe of the thyroid, it takes little (Simon) or no (Verdun) share in the formation of the 

 glandular substance. 



The thymus in the lower vertebrates appears as a series of buds from the 

 dorsal pockets of the branchial clefts. The number of buds differs in different 

 forms, and the number of those which persist and enter into the formation of the 

 adult gland varies. In birds and mammals it arises from one main rudiment 

 related to the ventral pocket of the third visceral pouch. To this is added in 

 birds and some mammals a rudiment from the fourth visceral pouch, but there 



