THE DEVELOPMENT OF THE THYMUS. 
BY 
EE. T. BELL; B:.S:;, M.D: 
Instructor in Anatomy, University of Missouri. 
With 3 PLATES AND 5 TEXT FIGURES. 
This paper is intended mainly as a contribution to our knowledge of 
the histogenesis of the thymus in mammals. Special attention is given 
to the origin and development of the corpuscles of Hassall, since their 
mode of formation has never been satisfactorily described in mammals 
and their significance in all forms is in dispute. An attempt is also 
made to show in detail the changes that occur during the transformation 
of the thymus from the epithelial to the lymphoid condition. 
This work was begun at the suggestion of Dr. D. D. Lewis at the 
University of Chicago. The greater part of it has been done at the 
University of Missouri. Special acknowledgments are due Dr. C. M. 
Jackson for valuable criticism and suggestions. I wish also to thank 
Mr. Charles H. Miller of the University of Chicago for his kindness in 
sending me material. 
MATERIAL AND METHODS. 
As material for the greater part of my work, I have used pig embryos 
from 8 mm. to full term (26 cm. to 30 cm.). These are especially suit- 
able for such work since they may be procured in abundance from the 
large packing houses at almost any stage of development. For special 
purposes I have studied a few specimens from human foetuses, and from 
the cat, rat, and guinea pig. The smaller pigs used (8 mm. to 27 mm. ") 
belong to the collection in the anatomical laboratory at the University of 
Missouri. These were stained in bulk with alum-cochineal and mounted 
in serial sections. In the later stages, which were prepared specially for 
this work, the ventral half of the cervical and anterior thoracic regions 
was usually cut out and embedded from pigs from 3 cm. to 8 em. On 
specimens from 8 cm. to 30 cm., I dissected out the thymus and used such 
parts as were desired. 
1The crown-rump measurement is used in all cases. 
AMERICAN JOURNAL OF ANATOMY.—VOL. V. 
