THYMUS-LIKE STRUCTURES IN AMMOCOETES 147 



structures with the chromatin generally collected into small 

 lumps situated next to the nuclear membrane. The nuclei are 

 smaller than those of the ciliated epithelial cells. Cell outlines 

 are more or less distinct in the placodes. A peculiar type of 

 vacuolization is in progress in some of the cells in which the 

 complete cell becomes vacuolated leaving the protoplasmic 

 remains as free bodies within the hollow cell. The proto- 

 plasmic bodies in these cases are small lumps of nucleated 

 protoplasm in which the nuclear material generally stains an 

 intense black and the cytoplasm a light red. These protoplasmic 

 bodies are not limited to the epithelial placodes, but may be 

 found everywhere in the branchial epithelium, especially in the 

 15 mm. larva. 



A layer of flat cells clothes the surface of the placode. This 

 layer is not distinct in every section and may easily be over- 

 looked. At the connective tissue border of the placode a base- 

 ment membrane sharply marks off the epithelium from the 

 mesoderm (b.rn., fig. 5). Lymphocytes have not been found 

 in the placodes in a 15 mm. larva although they are present in 

 the blood. They may be seen, however, in the general branchial 

 epithelium and also in the ciliated epithelium (Im., fig. 6). 



The placodes have increased considerably in size in a 31 mm. 

 larva. The mesenchymatous tissue which was present above the 

 placode in the 15 mm. stage has changed to connective tissue 

 (fig. 7). Large spaces containing various kinds of blood cells 

 are present in this connective tissue. Larger and smaller nuclei 

 may be seen in the walls of these spaces. The smaller undoubt- 

 edly represent the nuclei of endothelial cells. The larger, how- 

 ever, are apparently derived from the placode and are in a stage 

 of migration into the vascular spaces. It is doubtful whether 

 these spaces should be considered true blood channels. While 

 red blood cells are quite abundant in the spaces in this stage of 

 development they are practically absent in them in the full 

 grown larva. It is probable that they represent a primitive 

 type of lymph vessels, as has been suggested by various authors. 

 The connective tissue is of the fibrous variety in which the 

 individual fibers are quite slender. The fibers interlace to 



