The origin and histogenesis of the thymus in Raja batis. 471 



with the same function in any vertebrate animal ^). The thymus is 

 a paired metamerically repeated organ, if the gill-region be meta- 

 meric; but its development is confined to this region in all known 

 cases. 



It must, therefore, be held — and the contrary is impossible of 

 proof — that the thymus is the parent-source of all the 

 lymphoid structures of the body. This conclusion throws 

 light upon one of the teachings of embryologists, histologists, and 

 pathologists, that the thymus is an example of an organ, which, after 

 assuming function in early life, atrophies at a later period. This is 

 only certainly known to happen in mammals, and from it the inference 

 is drawn, that in later life the organ ceases to exist. 



It no more ceases to exist than would the Anglo-Saxon race dis- 

 appear, were the British Isles to sink beneath the waves. 



The simile is a real one, for just as the Anglo-Saxon stock has 

 made its way from its original home into all parts of the world, and 

 has there set up colonies for itself and for its increase, so the original 

 leucocytes, starting from their birthplace and home in the thymus, 

 have penetrated into almost every part of the body, and have there 

 created new centres for growth, for increase, and for useful work for 

 themselves and for the body. 



Literature cited. 



The following list of literature only contains that relating to the 

 Histogenesis of the Thymus. For the literature of its development 

 in various vertebrates, apart from its histogenesis, the reader may be 

 referred to the works of Prenant, Gulland, de Meueon, Kühn, Maurer, 

 LiviNi, and to Wiedersheim's "Vergleichende Anatomie". 



a) Works upon the Histogenesis of the Thymus. 



1894. Beard, J., The development and probable function of the Thymus, 



in: Anat. Anz,. V. 9, 1894, p. 476—486. 

 1900, 1. — , A Thymus-element of the spiracle in Raja, ibid., V. 18, 



p. 359—363. 



1) Apart from the hopeless quest, previously referred to, the only 

 other "organ" of this kind searched for has been the "mesoderm". In 

 embryology the mesoderm has only too frequently given rise, or ap- 

 peared to do so, to illegitimate progeny. 



