THE SYSTEMATIC ANATOMY OF A FCETAL SEA-LEOPARD. 459 



SECTION II. 



AN OUTLINE OF THE MICROSCOPICAL ANATOMY OF SOME OF THE ORGANS. 



The histological characters of certain of the viscera were examined in this foetus, 

 and in a few instances, viz. kidney and pituitary body, were found interesting from 

 the point of view of the histogenesis of their essential secreting portions. The 

 following account must, however, be looked upon as purely descriptive in character, 

 since from the mere observation of features presented by a single specimen at one 

 stage of development it is hardly possible to formulate a complete account of the 

 histogenesis of any one organ. 



(i.) The thyroid and parathyroids (see PI. III. fig. 2). The lateral lobes of the 

 thyroid gland are situated rather far forward in the neck region, reaching the level 

 of the lower border of the cricoid cartilage ; each lobe measures about half an inch 

 in length. The largest parathyroid is the posterior one, and is placed upon the 

 mesial surface of the thyroid at the ventral and posterior angle of the lobe. The 

 anterior parathyroid is quite small, and is found about the middle of the dorsal 

 margin of the lateral lobe. 



In minute structure the thyroid is seen to be made up of a large number of small 

 vesicles, some of them showing evidence of recent origin from branched tubular 

 columns of cells, and in some of them colloid is already to be detected ; the vesicles 

 are lined by a cubical epithelium, and there is a small amount of interstitial connec- 

 tive tissue, but no sign of a basement membrane outside the epithelium. The whole 

 lobe is subdivided into relatively few rather large lobules by coarser trabeculse of 

 connective tissue which are given off from the inner surface of a rudimentary 

 capsule which surrounds the lobes. Blood-vessels are fairly numerous, the larger 

 branches being supported by the coarser trabeculae of connective tissue lying 

 between the lobules. 



The parathyroid (PI. III. fig. 2, p.th.) shows a quite typical structure : it is 

 enclosed by a capsule of somewhat open connective tissue, which gives off trabeculae 

 passing into the interior of the gland. The essential secreting cells occur in the form 

 of folded columns of cuboidal or polyhedral cells, whilst these columns are in close 

 apposition to the blood-channels, the latter being at this stage lined by a definite 

 endothelium ; the latter, however, is not so marked a feature as it is in the blood- 

 channels occurring in the pars anterior of the pituitary body of this foetus. 



(ii.) The thymus shows on section and microscopical examination a number of 

 lobulated masses of lymphoid tissue supported by a framework of open connective 

 tissue in which run a few rather large blood-vessels ; here and there in the actual 

 substance of the lymphoid masses there occur a few areas which appear clearer and 

 which probably represent the remains' of the original lumina of the epithelial tubules 

 from which the gland arises. 



(ROY. SOC. EDIN. TRANS., VOL. L., 237.) 



