THYMUS-LIKE STRUCTUEES IN AMMOCOETES 149 



characters from the typical epithehal nucleus of the placode to 

 the mature lymphocyte-like cell. Nuclei showing these degrees 

 of variation may all be found in a single section. Figures 10 

 and 11 show the most obvious stages in this gradual variation. 

 The nuclei marked a are typical placode epithelium nuclei ap- 

 parently in a state of emigration. The nucleus marked b is in the 

 connective tissue. The chromatin in this nucleus is apparently 

 breaking up into a number of granules, a process which has 

 proceeded farther in nucleus c. In nucleus d the chromatin 

 gi-anules are apparently arranging themselves on the nuclear 

 membrane, an arrangement which has been completely attained 

 in nucleus e. Nucleus e further shows a tendency towards ac- 

 quiring a circular outline which becomes more manifest in 

 nucleus /. Nucleus / also shows a reduction in size. Nucleus g 

 (fig. 11) has a circular outline and further shows a change in the 

 character of the protoplasm. The nucleus h shows a still 

 further reduction in size, the protoplasm stains darker as does 

 also the chromatin. The chromatin, further, forms a continuous 

 layer at the periphery. In i (figs. 10 and 11) the nuclei have 

 acquired a thin covering of cytoplasm which is not visible at all 

 points of the nuclear surface. The cytoplasm stains a gray- 

 blue. The nucleoplasm and the chromatin of these cells take 

 the stain more intensely than the nucleus h. These cells have also 

 become free from the connective tissue mesh work. In the 

 cells j the nucleoplasm stains a deep purple. The chromatin 

 appears to have left the nuclear membrane and is now present 

 as granules scattered about in the nucleus. In some nuclei the 

 chromatin granules are connected together by slender processes, 

 in others this is apparently not the case. Still other cells show 

 nuclei in which the chromatin is represented by a single large 

 lump. These cells (j, figs. 10 and 11) represent the typical 

 lymphocyte-like cell in this region of the 31 mm. larva. Some 

 of these cells may be found in which the nucleoplasm stains a 

 gray-blue {k, fig. 10). They are similar to the lymphocytes in 

 older larvae and may either represent a final stage in the de- 

 velopment of the cells, or they may represent cells foreign to 

 this locality. 



