[THOMPSON] CERVICAL REGION OF THE FROG 67 
4, PosTBRANCHIAL Bopy. 
Historical. 
The subject of the post branchial body is of considerable importance 
on account of the relation of this organ to the development of the 
thyroid. De Meuron! was the first to describe the body in the frog and 
the toad and to discuss its embryology. He thought the structure to 
be homologous with the “suprapericardial body” which had been 
described by Van Bermmelen? in elasmobranchs. Maurer named it the 
post-branchial body to express its relation to the gill-clefts. In all 
vertebrates where it occurs it arises immediately behind the last gill- 
cleft, whether this be the fourth, fifth or sixth. (See text figure, p. 63). 
It has been found in all craniata except cyclostomes and teleosts; 
but in many forms it appears only on one side. 
Anatomy. 
The post branchial body in Rana pipiens is by no means easy to 
find with the unaided eye, though in some of the larger species of the 
anura it-may sometimes be found by dissecting off the mucous membrane 
from the floor of the mouth. The body is a tiny structure, flat and 
yellowish, which les some distance to the side of the aditus laryngis, 
very near the aorta (PI. I, fig. 1 pbb.) 
Histology. 
The organ consists of three or four small vesicles lined with 
epithelium (Pl. V, figs. 6 and 7). I have previously® described this as 
cylindrical. On renewed investigation it becomes clear that there 
several rows of nuclei and the appearance is that of a “ pseudo-stratified ” 
epithelium. Maurer says that these cells sometimes carry cilia. This 
I have not been able to observe. The vesicles contain a coagulated 
albuminous substance and debris, but no colloid. 
Development. 
The post branchial body arises on either side as an evagination of 
the ventral pharyngeal wall behind the fifth cleft, at the side of the 
aditus laryngis (Maurer, quoted by Gaupp!). (See text fig., p. 63). 

? Recueil Zool. Suisse III, 1886. 
? Mitt. a. d. zool. Stat. zu Neapel., Vol. 7, 2, 1885. 
? Phil. Trans. R.S. Series B, Vol. 201, p. 103. 
