FATE OF THE ULTIMOBRANCHIAL BODIES 91 



masses. This is brought about in an entirely passive way, by 

 the ingrowth of vascular tissue and of elements from the median 

 thyroid. These cell cords and cell masses stain more intensely 

 than do the elements of the median thyroid. Traces of the 

 lumen persist mainly in the more central part of the lateral thy- 

 roids. These structures in later developmental stages undergo 

 degenerative changes. This period, which he designates the 

 periode de survivance, is characterized by a disappearance of 

 the cell cords and the degeneration of the more centrally located 

 epithelial cells, forming cysts lined with cuboidal or columnar 

 epithelium which may or may not be ciliated. He also claims 

 that the formation of cysts in the lateral thyroids of pig embryos 

 is not a constant occurrence. Cysts in these structures in the 

 pig were found only in five out of eleven embryos which he exam- 

 ined. In a 210 mm. embryo, the largest examined, no traces of 

 the lateral thyroids were found. He is of the opinion that these 

 structures in the pig disappear entirely. 



Rabl ('08) finds that in the older mole embryos the lateral 

 thyroids are reduced to insignificant structures, being repre- 

 sented by cell cords and cysts. 



Verdun ('98) finds that in birds (chicken and duck) the post- 

 branchial bodies remain independent structures of a glandular 

 character but do not produce colloid. He regards these struc- 

 tures as special glands for birds. In the thyroid of mammals 

 (rabbit, cat, dog, mole) the postbranchial bodies are represented 

 by cysts and cell cords. The cysts vary greatly in size in the 

 different species studied. Neither during the embryonic nor 

 the postnatal life of these mammals was he able to demonstrate 

 the transformation of the epithelial cords of the postbranchial 

 bodies into thyroid follicles. He beheves that the cysts and 

 cell cords represent atrophied vestiges of the special gland in 

 birds. 



According to Tourneaux and Verdun ('97) the lateral thy- 

 roids in human embryos can for some time be recognized as a 

 rather densely staining mass on the posterior surface of the lateral 

 lobes of the thyroid gland. They undergo the same structural 

 changes as the median thyroid, but more slowly. From the 



