96 University of California Publications in Zoology 
4. The microscopic structure is suggestive of a lymph gland. 
A reticulum of connective tissue underlies the whole, and in this 
a number of other histological elements are netted. 
5. The most characteristic feature is the abundance of cells 
containing masses of granules staining deeply with eosin, of 
unknown nature, and resembling melanin granules in size and 
form. 
6. There is some evidence that the granules mentioned above 
pass into the blood vessels and there disintegrate. 
7. The earliest indication of the strueture is in an 18 mm. 
embryo, where there is a blood sinus lying between the covering 
of the fourth ventricle and the cartilaginous roof of the skull. 
This sinus is larger and more conspicuous in embryos 21.5 and 
22 mm. in length. 
8. Ina 55 mm. embryo the condition is directly intermediate 
between the 22 mm. stage and the adult. The connective tissue 
reticulum and pigment cells are well developed, and large blood 
sinuses are still present, but the granular masses so characteristic 
of the adult are not vet in evidence. 
