DEVELOPMENT OF THE THYMUS 4(35 



blood vessels and at the points of intersection of two or more 

 septa. In some places prolongations of the septa dip down into 

 the cortex of the lobules. These secondary septa approach 

 very nearly the medulla but seldom enter it and are usually 

 expanded at their deeper ends where they may lodge larger 

 blood vessels. The structure of the wider portions of the septa 

 is usually looser than the thinner parts. Small blood vessels of 

 a capillary nature are found through the septa and can often 

 be seen entering the cortex. Lymphocytes are present only in 

 comparatively small numbers. In the more compact portions of 

 the septa they may be entirely absent. 



A discussion of the red blood-cells and the granular leucocytes 

 in full term embryos will follow. 



3. The- epoch of the formation of the red hlood-cells and the develop- 

 ment of granular leucocytes 



Investigators disagree as to the extent of the formation of 

 red blood-cells in the thymus. Many have observed red blood- 

 cells lying free in the parenchyma of the thymus during both 

 its growth and involution but to my knowledge no extended 

 investigation through a wide range of developmental stages 

 has yet been made of their origin. Afanassiew (77) apparently 

 was the first to consider their origin. He held that during 

 the development of the thymus a rearrangement of some of 

 the blood vessels took place resulting in the formation of 

 the concentric (Hassall's) corpuscles. During this process 

 some of the blood vessels are ruptured thus permitting erythro- 

 cytes as well as leucocytes to escape into the parenchyma where 

 they then may be found singly or in groups. In mammals the 

 red blood-cells usually underwent degeneration. He regarded 

 the thymus a hemolytic organ. 



Watney ('82) also observed erythrocytes, 'hemoglobin masses,' 

 and cells containing fragments of hemoglobin in their cytoplasm 

 in the thymus of mammals, birds, reptiles, and fishes. He does 

 not state whether the erythrocytes and hemoglobin masses are 

 derived from cells in the parenchyma or whether they have 



