238 MR HAROLD AXEL HAIG ON 



The accessory portions of the thymus, noted in the general description of the 

 viscera as being isolated from the main mass, are in all probability derived from the 

 2nd branchial cleft, and do not fuse with the portions derived from the 3rd and 4th 

 clefts. 



Groschuff and Verdun state that the thymus in carnivora arises invariably from 

 the 3rd and 4th clefts, but in the rabbit, according to Verdun, additional parts may 

 arise from the 2nd cleft.* In the present instance, paired accessory portions were 

 found lying dorsal to the sterno-mastoid muscle on either side of the trachea, and 

 these on examination showed a typical thymus structure. 



The lymphoid nodules of the thymus present a more or less uniform density with 

 the exception of the occurrence of the above-mentioned clearer areas, and no sub- 

 division into cortex and medulla is as yet obvious ; nor are any corpuscles of 

 Hassall to be observed. 



(iii.) The lungs (PI. XXII. fig. 3). — Microscopical examination reveals in these 

 organs a structure entirely comparable to that of a compound tubular gland, the 

 branching tubules of which lie embedded in connective tissue ; the latter exists in 

 large amount and is of a fibro-cellular character. The epithelium lining all of the 

 ramifications of the bronchi is of the high columnar type, with the nuclei lying next 

 the basement membrane, whilst immediately outside the latter there is seen a fairly 

 wide zone of tissue, more densely cellular than the true interstitial connective tissue ; 

 this denser zone is the anlage of the fibro-muscular and elastic coats of the bronchioles. 



The epithelium of the branching tubules is ciliated, and it is only during later 

 stages that the cilia disappear in those portions of the bronchioles where the latter 

 expand into the infundibula and alveoli ; at the present stage, although in some places 

 the tubules appear to widen out into sac-like expansions, the epithelium remains of 

 the ciliated variety, since no true alveoli with air-sacs are as yet developed. 

 The interstitial connective tissue, which during later stages becomes compressed 

 and relatively diminished in amount by the preponderating development of the 

 alveoli, contains some large blood-vessels, but these are as yet relatively few in 

 number. 



(iv.) The pancreas (PI. XXII. fig. 5). — The histological features presented by this 

 gland are quite typical : branching tubules supported by a fine meshwork of con- 

 nective tissue, the whole enclosed by a capsule of somewhat open character, from 

 which trabeculse pass into the substance of the gland. The tubules are lined by 

 columnar epithelium, but there is as yet no definite basement membrane ; t the 

 lumina of the developing alveoli are quite small. The blood-vessels are quite small 

 and apparently not very numerous at this stage ; no signs of any cell-groups com- 

 parable to islets of Langerhans are to be detected, but probably it is too early for 



* See Hertwig, Handhuch der Entwichelungslehre der Wirbeltiere, 1906. 



t The. connective tissue appeared to have shrunken away from the tubules, leaving a considerable space between the 

 two (see fig. 5). 



