509 



pharynx and ectoderm, and probably also with the postbranchial the 

 body. It may be brought into relation with the thyreoid, but its 

 further history is unknown, — as is the early history of the para- 

 thyreoid bodies. Further information regarding the significance of the 

 postbranchial body is to be sought from comparative studies. In 

 selachians there is a structure found often on only one side of the 

 pharynx, behind the sixth pouch, and this is interpreted by both Van 

 Bemmelen and de Meuron ^) as a rudimentary seventh pouch. In mam- 

 mals they are inclined to assume that by the disappearance of the 

 fifth and sixth pouches the pecuharly modified and rudimentary seventh 

 has heen brought into relation with the fourth. But Maueer^) who 

 named the structure the postbranchial body writes that "It always 

 lies behind the last gill pouches, whether these are the fourth, fifth 

 or sixth. Therefore the organ is independent of the degenerating 

 pouches, and should be regarded as difiering from them radically." 

 Verdun^), because of its pecuhar properties, likewise considers it 

 wholly independent of the branchial pouches. Greil^) difi'ers with 

 these investigators : "The branchial nature of these bodies is established 

 beyond doubt." He names them the "ultimobranchial bodies" to indi- 

 cate that they represent the last pouches in the series, the seventh 

 in fishes, the sixth in amphibia, and the fifth in mammals. The fact 

 that the pulmonary arches are behind the fifth pouches in dipnoi, and 

 behind the postbranchial bodies in mammals, accords with the inter- 

 pretation of Zimmermann and of Greil that the postbranchial bodies 

 represent the fifth pair of pouches. 



Since so few reconstructions of both the vessels and pouches 

 have been published, the following series has been prepared from 

 rabbit and pig embryos preserved in the Harvard Collection. 



In the rabbit of 10 V 2 days, 4,4 mm, the fourth aortic arch is 

 complete, and a vessel leaving its ventral part (ventral aorta) extends 

 posteriorly, ending blindly under the expanded portion of the ento- 

 dermal tract. This expanded part represents the pharynx. Its most 

 posterior and lateral corner marks the position of the postbranchial 

 body. No fourth pouch has appeared. The third pouch, not included 

 in these reconstructions, is well developed, anterior to the fourth arch. 

 Its ventral arm which forms the thymus, extends toward the angle of 



1) P. DE Meurox, Recherches sur le developpement du thymus 

 etc., Geneve 1886, p. 100 et seq. 



2) P. Maurer, 1. c. p. 142—146. 



3) P. Verdun, 1. c. p. 188 et seq. 



4) A. Greil, Anat. Hefte, Bd. 29, 1905, p. 445—506. 



