THYMUS-LIKE STRUCTURES IN AMMOCOETES 151 



nuclei. The nuclei at the border show a tendency to stain blue, 

 the chromatin takes a darker stain, and in some cases is broken 

 up into granules, and the nuclei approach the globular shape. 

 The transformation of the nuclei in the connective tissue appears 

 to be of the same character as in the 31 mm. stage, but appar- 

 ently more rapid. Nuclei may occasionally be found in the plac- 

 ode which show phagocytic properties. Figure 14 represents a 

 placode nucleus in the act of engulfing protoplasmic bodies. 



The spaces in the connective tissue in the 44 mm. larva appear 

 to be smaller than in the 31 mm. stage. Some of them have a 

 distinct wall while others appear like transient spaces in the 

 connective tissue. Red blood cells are only occasionally seen 

 in the connective tissue spaces of this stage. 



The placodes in a 63 mm. larva are larger in area but thinner 

 than in the preceding stages. The nuclei of the epithelial cells 

 of the placode have lost their original character. The chromatin 

 is no longer represented by a single large lump, but is present 

 in the form of granules corresponding to the chromatin in the 

 nuclei which had migrated into the connective tissue in the 31 

 mm. stage. The number of lymphocyte-like cells has increased 

 considerably within the placode. All the stages of transforma- 

 tion from epithelial nucleus to the mature lymphocyte-like cell 

 may be found within the placode in this stage of development. 

 A basement membrane is re-forming at the connective tissue 

 border of the placode. The 'vascular spaces' of the connective 

 tissue are now chiefly limited lo the peripheral part of the whole 

 connective tissue within the ventral half of the epipharyngeal 

 ridge. The mature lymphocyte-like cells are chiefly located in 

 these channels, leaving the central connective tissue core quite 

 free from cells. The central core consequently has a much 

 lighter appearance. Some nuclei are present in the central core, 

 the morphological characters of which are similar to the charac- 

 ters of connective tissue nuclei in other parts of the body. 

 Other nuclei may occasionally be seen in which the characters 

 agree with the various transformation stages of the lympho- 

 cyte-like formation shown in younger larvae. 



