280 THE DUCTLESS GLANDS 



From the epithelium on the ventral aspect of the third 

 cleft arises the main portion of the thynius. This may be 

 called "thymus III." (see Fig. 82, p. 281). The ventral 

 proliferation of the epithelium of the fourth cleft also gives 

 rise to thymus tissue, but this is usually simply a small nodule 

 in the substance of the thyroid, and is called " thymus IV." 



The dorsal aspects of the third and fourth clefts show 

 thickenings of the epithelium which, however, do not develop 

 into thymus tissue, but retain a typical " epithelial " struc- 

 ture. These are the parathyroids named respectively " para- 

 thyroid III." and "parathyroid IV." In connection with 

 the last is found a cavity lined with epithelium. This is the 

 post-branchial body. 



The thymus nodule sometimes found in close relation to 

 the external parathyroid ("parathyroid III.") is simply an 

 isolated portion of the main thymus ("thymus III."). 



The accompanying diagrams (from Kohn) will help to make 

 the matter clear (Figs. 82, A and B). 



EXPLANATION OF FIG. 82. 

 (Diagrams A and B.) 



A. illustrates the development of the branchial organs of mammals, B. shows 

 their actual relations in the adult. 



The different related rudiments of the same branchiomere are represented 

 by a similar direction of shading lines ; so also the corresponding organs. Thus 

 the rudiments from the third cleft are represented in A. by horizontal lines, 

 as also the organs thus arising in B. The rudiments and organs from the 

 fourth cleft are characterized by vertical lines. The post-branchial body is 

 shown in thick outline, the thyroid by crossed lines. 



The different kinds of tissue arising from one and the same branchiomere 

 are indicated by differences in the shading. The parathyroid tissue is shown 

 by lines, the thymus tissue by alternate continuous and interrupted lines. 

 The post-branchial body is represented in the developed condition as a hollow 

 space with several glandular nodules (shown in dark circles). 



A. shows the four internal gill slits (I. to IV.), the epithelial origins of the 

 parathyroids (p. thyr. III., p. thyr. TV.), the origin of the thymus (thym. III., 

 thym. IV.), the rudiment of the thyroid (thyr.), and that of the post- bran oliial 

 body (p. b. b.). 



B. represents a schematic transverse section through the fully developed 

 thyroid (at about the level of the junction of the upper and middle third of the 

 thyroid lobe of a cat). The lettering corresponds to that in Diagram A. The 

 structures which arise from the third cleft become the " external " para- 

 thyroid and thymus nodule (separated fragment of thymus III. ) ; those which 

 arise from the fourth cleft become the " internal " parathyroid and the thymus 

 nodule (p. thyr. IV., and thym. IV.). The post-branchial body is surrounded 

 by thyroid tissue. 



(A. is after Groschuff in ruminants ; B. from Kohn in the cat.) 



