364 



E. Y. COWDKY 



thymic tissue lias been removed. Each lobe of the definitive thynms is 

 thus formed from a solid cord of cells. Up to this point the gland is en- 

 tirely epithelial. 



Differentiation and Growth. The growing differentiation into cor- 

 tical and medullary portions becomes first apparent in embryos of about 



Fig. 3. Diagram showing derivatives of branchial pouches; thyroid (t) and 

 ultimobranchial bodies (u) in horizontal lines, parathyroids (p) in vertical lines and 

 the thymus (th) dotted, after Grosser, modified. 



forty-five millimeters. The small round cells become more and more 

 abundant, especially in the part destined to give rise to the cortex. Ac- 

 cording to Hammar (&) (c), the Hassall's corpuscles appear in embryos 

 of from sixty to seventy millimeters. 



The growth changes are indicated in the following table: 



TABLE 2 



GROWTH CHANGES IN THE THYMUS GLAND, AFTER HOSKIXS, SLIGHTLY MODIFIED 



