DEVELOPMENT OF THE THYMUS 485 



8. It was impossible to trace the origin of all the eosinophile 

 granules in the eosinophile cells directly to degenerated red 

 blood-cells. However, the fact, that the height and decrease 

 of the formation of red blood-cells in the septa is followed by the 

 height and decrease of the formation of eosinophile cells, is cir- 

 cmnstantial evidence that a relationship exists between the 

 disappearance of the free erythrocytes and the formation of 

 free eosinophile cells. 



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

 each of which is characterized by more or less distinct develop- 

 mental features. They are: 



(1) The purely epithelial epoch which extends from its origin 

 as an outpocketing from the third pharyngeal pouch and the 

 formation of the cervical vesicle to the appearance of the first 

 lymphocytes in the thymus. 



(2) The epoch of lymphocyte infiltration and lymphocj^te 

 proliferation and the formation of the reticulum. The in- 

 filtration of the thymus by extra thymic lymphocytes from the 

 mesenchyme surrounding it begins in embryos from 25 to 30 

 mm. in length and probably continues up to stages 180 mm. in 

 length, while their proliferation in the thymus undoubtedly 

 continues after birth. The reticulum, which according to the 

 nature of its development is formed gradually, differentiates 

 into the cortex and the medulla in developmental stages 65 to 

 75 mm. in length, and is fully formed in embryos 180 mm. in 

 length. 



(3) The epoch of the formation of red blood-cells and the 

 development of granular cells. An occasional red blood-cell is 

 found in the thymic lobules shortly after lymphocytes are found 

 in them. They are, however, first present in appreciably large 

 numbers in stages of about 55 mm. in length and are most numer- 

 ous in the thymus of full term embryos. In the interlobular 

 septa of the thymus the greatest number occurs in stages of 

 about 125 mm. in length while only a few are found in embryos 

 of 180 mm. in length to full term. 



Eosinophile cells were first found in the thymic lobules of a 

 42 mm. embryo but occur first in appreciably large numbers 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 17, NO. i 



