458 SUMMABY OF CURRENT RESEARCHES RELATING TO 



respective origin of each segment is described. (1) The superficial 

 thymus, which is a derivative of the cervical vesicle, has a purely 

 ectodermal origin. (2) The connecting band is also a derivative of 

 the cervical vesicle, and has, therefore, a purely ectodermal origin. In 

 most embryos it persists to birth, but it may be absent on both sides or 

 on one side. (3) The thymus head, in which is lodged the parathyroid 

 III, is formed by a fusion of a portion of the cervical vesicle to the 

 anterior end of the epithelial diverticulum derived from the third 

 pharyngeal pouch. It has, therefore, an ectodermal-endodermal origin. 

 (4) The intermediary and cervico-thoracic cords, and the mid-cervical 

 and thoracic segments, are derived wholly from the epithelial diverticulum 

 of the third pharyngeal pouch, and have, therefore, a purely endodermal 

 origin. 



Development of Thymus in Pig.* — J. A. Badertscher comes to the 

 following conclusions in regard to the histogenesis of the thymus in the 

 pig. The lymphocytes first present in the thymus are all large lympho- 

 cytes, and have migrated into it from the mesenchyme. The numerous 

 small round cells of the thymus are formed by the repeated division of 

 the large lymphocytes, which thus become small, and also by their own 

 proliferation. From the source and structure of the small round cells 

 it is inferred that they are small lymphocytes and identical with the 

 small lymphocytes of the blood. The thymus, therefore, may well be 

 considered as a source of some of the small lymphocytes found in the 

 circulating blood. The reticulum of the thymus is of epithelial origin, 

 and is formed passively by its meshes becoming filled with lymphocytes 

 which separate the nodal nuclei farther apart, and thus greatly attenuate 

 the protoplasmic processes of the syncytium. The Hassall's corpuscles 

 are of epithelial origin. The free red blood-cells and eosinophile cells 

 found in the interlobular septa and the thymic lobules are derived from 

 lymphocytes in situ. It is difficult to determine whether or not any of 

 the erythrocytes formed in the thymus enter the circulating blood. 

 Some undergo degeneration, and the products of disintegration of those 

 existing in the form of eosinophile granules are taken up by the lympho- 

 cytes, which thus become transformed into eosinophile leucocytes. It 

 was impossible to trace the origin of all the eosinophile granules in the 

 eosinophile cells directly to degenerated red blood-cells, but a relationship 

 exists between the disappearance of the free erythrocytes and the forma- 

 tion of free eosinophile cells. 



The histogenesis of the thymus may be divided into three epochs. 

 1 . The purely epithelial epoch extends from the origin of an outpocket- 

 ing from the third pharyngeal pouch and the formation of the cervical 

 vesicle to the appearance of the first lymphocytes in the thymus. 



2. Second, there is the epoch of lymphocyte infiltration and lympho- 

 cyte proliferation and the formation of the reticulum. The infiltration 

 of the thymus by extrathymic lymphocytes from the mesenchyme 

 surrounding it begins in embryos from 25-80 mm. in length, and 

 probably continues up to stages 180 mm. in length, while their pro- 

 liferation in the thymus undoubtedly continues after birth. The 



* Amer. Journ. Anat., xvii. (1915) pp. 437-92 (3 pis.). 



