262 THE DUCTLESS GLANDS 



up of structures identical with the various well-known forms 

 of HassaU's concentric corpuscles of the thymus. There is a 

 mass of this tissue in the centre of the parathyroid, and small 

 portions of it in other regions of the glandule. There are also 

 true thymus nodules in close connection with the body. Thus 

 the structure which is commonly called the post-branchial 

 body in birds (a yellowish-white body some little distance 

 posterior to the thyroid), is composed not only of post-bran- 

 chial tissue, but also of parathyroid and thymus. Although 

 a parathyroid is derived from both third and fourth clefts, 

 yet the one derived from the latter does not become enclosed 

 in the thyroid tissue as is the case in mammals. 



8. Mammalia. (a) The general position and anatomical 

 relations of the thyroid in mammals is too well known 

 to call for a description. It will, however, be useful to 

 describe in a series of mammals the probable number and 

 the usual position of the parathyroids in relation to the 

 thyroid. 



The parathyroids of mammals are usually four in number. 

 They are small, oval, or spherical bodies, in most cases of a 

 distinctly lighter tint than the thyroid tissue, and occupy 

 very variable positions not only in different species, but in 

 different individuals. On either side of the median line of 

 the body there are typically two of these glandules, which 

 are referred to at the present time either as " external " and 

 " internal " or as " parathyroid III " and " parathyroid IV " 

 respectively, the Roman numeral indicating the number of 

 the gill-cleft from whose epithelium the gland was originally 

 formed. The latter mode of signification is by far the most 

 precise, but the majority of writers prefer to refer to an external 

 and an internal glandule on either side. The terms " external ' ' 

 and "internal" appear, however, to be used by different 

 authors in different senses. Kohn's definition of the terms is 

 given in the following words : " Das eine lag in der Regel der 

 Aussenflache der Seitenlappen lose an das andere innerhalb 

 derselben. Ersteres wurde ' ausseres' letzteres ' inner es ' . . . 

 gennant." By "external" and "internal" here is implied 

 "on the surface of" and "in the interior of" respectively. 

 But Schafer interprets the terms differently. Thus, " there 

 is one parathyroid (' outer epithelial body ') constantly to be 

 met with in mammals on the lateral surface of each lobe of 



