DEVELOPMENT OF THE THYMUS 439 



28, 30, 33, 35, 36, 37, 40, 42, 45, 50, 50, 55, 60, 65, 68, 78, 85, 100, 

 110, 115, 125, 135, 140, 165, 180, 190, 210, 230, 270, and 280. 

 These figures represent the length of the embryos while in a 

 fresh condition. 



Helly's fluid (Zenker-formol) was aknost exclusively used 

 for fixing the material. This fixer does not destroy the baso- 

 philic character of the cytoplasm of the lymphocytes and appar- 

 ently produces no appreciable alteration in the hemoglobin of 

 the red blood-cells found in the thymus. A few embryos of 

 different developmental stages were fixed in Zenker's fluid, 

 mainly to check up the results of the work done by investigators 

 who emploj^ed this fixer for a histogenetic investigation of the 

 thymus. It was found that Zenker's fluid is not at all suitable 

 for work of this nature since, to a large extent, it destroys the 

 basophilic character of the lymphocytes, the preservation of 

 which is of inestimable value in tracing out the origin of the first 

 lymphocytes found in the thymus. The tissue was mibedded 

 in paraffine and cut in sections 3 to 5 /x in thickness. Only such 

 sect'ions as were desired were spread on slides. • These always 

 included sections of the superficial thymus, the thymus head, 

 and cervical and thoracic segments.' For the preservation of 

 cells with basophilic granules the material was fixed in 95 per 

 cent alcohol. 



Basting's modification of Nocht's Romanowsky blood stain 

 proved to be of the greatest value for this work and was almost 

 exclusively used in the investigation of the histogenesis of the 

 thymus. In properly differentiated sections the cytoplasm 

 of the lymphocytes, which has a distinctly basophilic character, 

 stains a light blue, while the cytoplasm of the epithelial cells 

 of the thymus and that of the mesenchymal cells stains a light 

 red color. The Ijonphocytes can thus be distinguished from the 

 epithelial nuclei with comparative ease. Erythrocytes and the 

 granules of eosinophile cells are stained intensely red, while 

 the granules of the cells with basophilic granules (fixed in 95 

 per cent alcohol, are stained blue. In tissue fixed in Zenker's 



1 For a discussion of the different regions of the thymus in the pig reference 

 should be made to Part I of this investigation ('14). 



