The origin and histogenesis of the thymus in Raja batis. 453 



fortunate circumstance, that he was able to investigate them at once 

 in a series of foetuses in the possession of his friend and former 

 pupil, Dr. J. A. Murray. These extend from about 22 days to the 

 end of the gestation, 66 days, and they had been obtained by Murray 

 for other purposes. For his kindness in placing them at my service 

 my best thanks are due, and may be here expressed. 



In late foetuses and new-born cavies the thymus does not occupy 

 the position taken up by it in front of the pericardium in man and 

 rabbit, etc. Neither are the portions of the two sides fused together, 

 as described, for instance, by Keibel in the pig. In certain marsupials 

 Symington (1898) has recorded the fact, that there the thymus lies 

 partly in the neck and partly in the thorax. 



On dissecting new-born cavies for the thymus Dr. Murray and 

 the writer were at first at a loss as to its whereabouts. We saw 

 what for a moment we took to be saHvary glands of large size on 

 each side of the neck^). On examining these it was found, that in 

 spite of their large size and unusual position, on each side of the 

 neck some distanee behind the angle of the jaw, we had in them the 

 two "thymus-glands" of the cavy. 



The forms studied have been specimens of 22, 31, 42, 55, and 

 66 (new-born) days cavies. In the 22 days embryo of the critical 

 period there were no traces of concentric corpuscles. In the 31 days 

 foetus, and, of course, in all the later ones, concentric corpuscles 

 were well represented. The period of their first appearance in the 

 cavy, therefore, is somewhere between the 22. and 31. days. It had 

 been intended to have obtained foetuses of this period, in order to 

 establish the exact epoch, but two considerations put an end to this 

 idea. As the cavy-preparations studied amply proved, the concentric 

 corpuscles are degenerate structures, of no morphological importance, 

 and they only appear in the cavy long after most of the original 

 epithehal cells have been converted into leucocytes. Soon after this 

 had been established, the facts, concerning the degeneration of certain 

 germ-cells, as elsewhere recorded, were made out, and in the light of 

 these there seemed no great unlikelihood of the degeneration of leuco- 

 cytes, owing to pluripolar mitoses, by the formation of cell-nests or 

 concentric corpuscles. 



1) It was only afterwards noted, that Watney in his memoir, 

 cited on a later page, had recorded this position of the thymus in the 

 cavy. He neither figured nor described the concentric corpuscles in 

 this animal. 



