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 Uebersetzungsrecht vorbehalten. 



The Origin and Histogenesis of the Thymus 

 in Eaja hatis. 



By 

 John Beard, D. Sc, 



University Lecturer in Comparative Embryology, Edinburgh. 



With plates 5 — 10 and 8 tezt-figare^. 



Table of Contents. 



Introduction. 



Table of Embryos, their Sizes preserved, and the Figures from them. 

 I. The early History of the Thymus-Placodes. 

 II. The Morphological Nature of Leucocytes. 



III. The Origin of the first Leucocytes. 



IV. The Thymus-Placodes in Embryos of 19 — 23 mm. 



V. The Mode of Formation of Leucocytes from epithelial Cells of 



the Placodes. 

 VI. The Thymus-Placodes in Embryos of 24 — 27 mm, 

 VIL The Thymus-Placodes in Embryos of 28—36 mm. 

 VIII. The later History of the Thymus-Placodes (Embryos of 37 — 70 mm). 

 IX. The History of the spiracular Thymus-Placode. 

 X. Thymus-Placodes and sensory Placodes. 

 XL Hassall's concentric Corpuscles of the Thymus of the Cavy. 

 XII. Historical. 

 XIII. The Thymus as the Source of Leucocytes. 

 Literature cited. 

 Description of Plates. 



Introduction. 



The researches, detailed in the following pages, were begun in 

 the spring of 1898, and brought to a preliminary close before the 

 end of the same year. They were continued, at first upon the old 

 material and afterwards upon a new set of specimens of Raja hatis, 

 during 1899, and, with the brief study of Hassall's concentric cor- 

 puscles in a series of cavies, they were finished, as here published, 

 early in 1900. 



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