146 IVAN E. WALLTN 



similar channels within the body of the gill as well as with the 

 definitive blood vessels of the pharynx. 



Lymphocytes begin to make their first appearance in the blood 

 in larvae of about 9 mm. length. They increase in number 

 with the growth of the larvae, but chiefly remain outside of the 

 main blood vessels. They are especially abundant in the peri- 

 vascular spaces of the gill arches. 



The accumulations of lymphocytes in the lateral branchial 

 walls are foreign to these situations so far as their origin is con- 

 cerned. Furthermore, the epithelial placodes in these situations 

 together with the lymphoid accumulations do not exhibit the 

 characters which are essential in either a well-established or 

 rudimentary thymus. I can offer no suggestion in regard to any 

 special significance of these accumulations. They appear to be 

 merely a part of a rich accumulation of lymphocytes in the con- 

 nective tissue spaces of the branchial region. It is probable 

 that the apparently degenerating placodes play a role of attrac- 

 tion for lymphocytes. 



Epipharyngeal placodes 



The placodes in the epipharyngeal ridge are present in an un- 

 differentiated condition in a 15 mm. larva. They form distinct 

 masses of cells in the mid-ventral part of the epipharyngeal ridge 

 between the ciliated bands. They are very nearly circular in 

 outline in a transverse section, producing a bulging into the 

 interior of the ridge (figs. 5 and 6). A loose mesenchymatous 

 tissue caps the dorsal surface of the placodes. Red blood cells 

 are occasionally present in the spaces of the mesenchymal 

 tissue. These spaces are apparently in communication mth the 

 dorsal aorta by means of minute apertures and are directlj^ con- 

 tinuous with siixdlar spaces in the connective tissue of the gill 

 arches. Through the spaces of the gill arches a communication 

 is also made with the peribranchial sinuses and the perivascular 

 spaces in the gills. 



Within the placode the cells are in an active state of pro- 

 liferation. The nuclei of the resting cells are rather clear 



