I04 BIOLOGY OF THE LABORATORY MOUSE 



Toward the medulla the density of the thymocytes changes rather 

 suddenly and the light staining reticular cells become much more promi- 

 nent. These cells are entodermal in origin and their epithelial character is 

 evident during embryonic life. The reticular cells have pale spherical 

 nuclei with fine chromatin granules and indefinite cell outlines. Thymo- 

 cytes are present, although not densely arranged. Eosinophils and plasma 

 cells are usually found here. Small groups of large, pale staining epithelioid 



Capsule 



Cortex 



cells with large oval nuclei occur in the medulla. These, however, are not 

 flattened and concentrically arranged like the typical Hassall's corpuscles. 

 Cysts of varying sizes are frequently present. The smaller cysts are often 

 lined entirely by cuboidal, ciliated cells; in the larger ones the lining con- 

 sists partially of fiat, partially of ciliated cuboidal cells. 



The gland is relatively large during embryonic life and in postnatal life up 

 to the time of puberty but undergoes involution after sexual maturity is 

 reached. During involution the thymic cells of the cortex gradually become 

 less dense and the division into cortical and medullary part is less prominent. 

 Adipose tissue is not deposited in the involuted thymus of the mouse. 



Arteries enter the capsule, are distributed first to the cortex, then to the 

 medulla. Veins arise in the medulla and leave the organ at the hilus. 



Pineal body. — The pineal body (epiphysis cerebri) is a small, cone-shaped 

 body which is situated above the roof of the third ventricle and is attached 

 to its posterior part by a stalk. The gland has a fibrous connective tissue 



