CIRCULATORY SYSTEM. 



129 



ileum they are collected into patches called agmi- 

 nated lymph nodules or Peyer's patches. 



Hemolymph Glands. These resemble the lymph 

 nodes described above, except that the lymph sinus 

 is filled with blood. When first discovered they were 

 believed to be evidence of disease, but they are now 

 looked upon as normal structures. They are most 

 readily found in the fascia involving the thoracic 

 aorta, and are particularly abundant in the sheep. 



Fig. 93. Section of lobule of thymus gland. 



THYMUS GLAND. 



The thymus gland is described in this place be- 

 cause of its resemblance in the adult to a lymph 

 organ. In the embryo it is an epithelial organ that 

 develops from the hypoderm of the third and fourth 

 visceral clefts. Lymphoid tissue invades this epithe- 

 lium and reaches its highest development in a child 

 two years old. After this age the lymphoid tissue is 

 9 



