374 



is 6 mill long and about 19 days old and was cut transversely and 

 reconstructed. 



Immediately behind the pituitary diverticulum (Pit. Fig. 1 a) 

 in the dorsal wall of the pharynx, there is a second small median diver- 

 ticulum (P. E.) extending upwards through six or seven sections and 

 ending in a solid rod of cells which continues for six more. The cells 

 composing this rod are identical with those of the thickened pharyn- 

 geal epithelium which forms the walls of the diverticulum and are con- 

 tinuous with the latter (Figs. 2 and 3). The whole structure runs 

 through 13 sections. The notochord ends 12 sections behind the 

 point at which the diverticulum appears in the pharyngeal roof, and 



^^q 



PR_ 



e>c «®o 



Fig. 4. High power. P. Pharynx. F.R. Recess. 



bends down before it does so that it ends almost touching the wall 

 of the pharynx. It resembles most closely the rod in which the 

 diverticulum ends, the only difference being that while the notochord 

 is enclosed in a definite canal, the solid end of the diverticulum lies 

 in the mesoderm tissue, which forms a clear space around it (Figs. 3 

 and 5). The diverticulum is continuous posteriorly with a broad 

 median groove, which runs for some distance. The epithelium of this 

 part of the pharyngeal wall is thickened as it is in the region of the 

 recess itself. In the walls of the recess there is marked proliferation 

 of these cells and of the mesoderm surrounding them, constituting 

 a rudimentary formation of lymphoid tissue. (Figs. 4 and 5). The 

 rod in which the recess ends consists of cells grouped circularly and 

 has a clear centre but no definite lumen (Fig. 3 b). Similarly the sur- 

 rounding mesoderm cells are thickened and proliferating and grouped 



