26 CHARLES EUGENE JOHNSON 



REVIEW OF LITERATURE 



The results obtained by different investigators of the devel- 

 opment of the thymus in the chick vary. Van Bemmelen ('86) 

 and DeMeuron ('86) came to the conclusion that the thymus here 

 is derived from the third and fourth gill clefts. Kastschenko ('87) 

 held that it is formed not only from the third and fourth gill 

 pouches, but also from the fifth, while Mall ('87) asserted that 

 the gland is a product of the third visceral pouch only. More 

 recently the subject has been thoroughly investigated by Verdun 

 ('98) who found the thymus in the chick to arise from the third 

 and fourth visceral pouches, the derivative of the latter pouch 

 being somewhat rudimentary. Keibel and Abraham (quoted 

 from Helgesson) in their Normentafel, 1900, also show the thy- 

 mus of the chick as a derivative of the third and fourth pouches. 



In the duck and the sparrow, according to the studies of Ham- 

 ilton and Helgesson respectively, the thymus is formed from the 

 third visceral pouch only. 



Parathyreoid bodies occur in all species of birds investigated. 

 According to Van Bemmelen, De Meuron, and Mall, in the chick, 

 the third and the fourth visceral pouches each gives rise to a para- 

 thyreoid body, and the same has been found true by Hamilton 

 and Helgesson for the duck and the sparrow. Verdun, in the 

 chick, found a third parathyreoid body arising in connection 

 with the postbranchial body; this was of rather rudimentary 

 nature and enclosed within the postbranchial organ. It has 

 been interpreted as representing a parathyreoid derivative of the 

 fifth visceral pouch. 



The postbranchial bodies arise as paired structures behind 

 the fourth visceral pouches, and, with the exception of Rabl ('07), 

 whose work will be considered later, are generally held to be de- 

 rivatives of the fifth visceral pouches in birds. In the chick, 

 according to Mall, the postbranchial bodies, which at first form 

 epithelial vesicles, gradually break down after the tenth day, . 

 and at the end of the period of incubation, form the mass of gran- 

 ules which ''Remak saw surrounding the fourth aortic arch." 

 Verdun, however, states that at about the tenth day these bodies 



