444 J. A. BADERTSCHER 



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

 ment of granular leucocytes. Although an occasional red blood- 

 cell is formed in the thymus shortly after the appearance of 

 the first lymphocytes, this epoch properly begins in embryos 

 about 55 mm. in length for it is at this developmental stage that 

 erythrocytes are beginning to be formed in comparatively large 

 numbers. Granular cells appear first in appreciably large 

 numbers in embryos 125 mm. in length. The formation of both 

 erythrocj^tes and granular cells in the thymus still continued in 

 the full term embryo. 



1. The purely epithelial epoch 



A 23 mm. embryo is the first stage in which the histological 

 structure of the thymus will be described. The thymus at this 

 stage is a purely epithelial structure. It has the form of a greatly 

 elongated mass of protoplasm and is a syncytium. No cell 

 walls are present. The cytoplasm of the superficial thymus, 

 the thymus head, and the mid-cervical and thoracic segments 

 contain many vacuoles which vary in size anjovhere from the 

 (apparent) size of a small pinhead to that of an epithelial nucleus 

 when magnified 1300 diameters. The vacuoles in the inter- 

 mediary and cervico-thoracic cords are comparatively few in 

 number. Fine, branching, and rather deeply stained proto- 

 plasmic threads give the syncytium a distinctly reticular appear- 

 ance. These protoplasmic threads, however, must not be con- 

 fused with the reticulum in later developmental stages. The 

 outer surface of the enlarged portions of the thymus is already 

 quite irregular, being studded over with blunt epithelial buds 

 which are the beginnings of lobules. No basement membrane 

 is present. 



The form and size of the epithelial nuclei vary considerably. 

 While the majority are slightly ellipsoidal in shape, some are 

 spherical and others slightly irregular in outline. They are 

 quite regularly distributed through the different segments of the 

 organ, lying farther apart in the more vacuolar regions than in 



