DR. H. WATNEY ON THE MINUTE ANATOMY OF THE THYMUS. 
1073 
History of the views concerning the development of the thymus. 
This part of the subject has been given separately. 
Haugsted (1), 1832, found that the thymus appears in the ninth or tenth week in 
the human embryo, as a double organ. 
Arnold (74), 1831 and 1837, thought that the thymus is developed from the mucous 
membrane of the respiratory tract, and that it arises nearly in common with the larynx 
and thyroid. 
Bischofe (28), 1842, noticed the gland as early as the eighth week in the Mam¬ 
malian embryo ; he considered that it arises in connexion with the thyroid. 
Simon (2), 1845, described and figured the thymus as commencing as a simple 
hollow tube; the tube bulges at certain points, and gives origin to diverticula or 
follicles, and by the extension of this process of follicular growth, the thymus attains 
its bulk and complexity of structure. 
Ecker, 1851-1859, in the ‘leones Physiologicae,’ gave drawings of the position of 
the thymus in- the Chick on the nineteenth day of incubation, and in the embryo of 
Squatina vulgaris. 
Hem an (57), 1855, said that the thymus is developed from part of the pharyngeal 
wall, and arises between the arches of the aorta; that in the front of the neck of the 
Chick there is a gland which disappears very rapidly. He found that it has near rela¬ 
tions to the thymus, although the one is not developed from the other; he accepted 
Ecjker’s view of the position of the thymus in the neck of the Chick. 
Friedleben (3), 1858, found that the gland is not formed originally as a tube, but 
that it appears as a connective-tissue stroma, in which round nuclei are imbedded. 
This stroma multiplies on all sides ; the capsule then shows traces of blood vessels. 
Robin (75), 1874-1875, said that by the successive invaginations of the internal folds 
(? of the hypoblast) the epithelium of the pharynx, of the cesophagus, of the trachaea, 
and of the lungs is formed; and that the thymus and the thyroid are produced in the 
same manner. 
Afanassiew (33), 1877, found that the thymus is developed in the Chick at the 
end of the fifth or sixth day of incubation in the reticular connective-tissue, which lies 
behind and outside the vessels, as masses of indifferent cells. In a foetal Rabbit, one- 
fifth of an inch in length, he saw the commencing thymus in the connective-tissue in 
front of the carotid artery, as a heaping up of indifferent cells. 
Dahms (68), 1877, thought that the thymus arises as a mass of agglomerate little 
rounded bodies around a clearer part : that in some cases there is a canal in the very 
early stage ; and that there is an invagination of the mucous membrane of the respira¬ 
tory tract. 
Kolliker (76), 1879, said that it arises, as an epithelial organ, from the branchial 
clefts; he gave a drawing of a cross-section of the gland, in its earliest stage of develop¬ 
ment, showing a central canal. The canal is lined by many layers of epithelial cells. 
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