458 J. A. BADERTSCHER 



division give rise to the small lymphocytes. This view of the 

 origin of the small lymphocytes in the thymus is in accord with 

 that of Hammer for teleosts ('08) and with that of Maximow 

 for mammals ('09 b). 



The mesenchyme of the interlobular septa (S.i.) in the head 

 and thoracic segments contains a large number of large and 

 intermediate sized lymphocytes. A few small lymphocytes 

 are also present. The mesenchyme surrounding the above 

 named regions also contains a relatively greater number of 

 lymphocytes than it does in corresponding regions of the pre- 

 vious stage described. 



An occasional nucleated red blood-cell can be found lying free 

 in the thymus. At this stage they are scarcely more numerous 

 than in the previous stage. None were present in that part 

 of the section from which the figure was drawn. Eosinophile 

 cells also can be found occasionally in any part of the mesen- 

 chyme. They were first found in embryos 35 mm. in length. 

 None were seen in the thymus. 



Embryo of 66 mm. (fig. 4)- The lobules are more numerous 

 than in the previous stage along the entire extent of the organ. 

 Those of the enlarged regions of the thymus are much more 

 voluminous than those of the intermediary and cervico-thoracic 

 cords. An almost interrupted layer of greatly attenuated mes- 

 enchymal cells closely invests the outer surface of the lobes and 

 apparently forms a limiting membrane (fig. 3, L.M.) for the 

 outer surface of the thymus. This membrane is present in 

 slightly earlier stages. Blood vessels {Bl.V.) are numerous in 

 the interlobular septa. No thick walled vessels are yet present. 

 The walls of most of them are made up of endothelium only. 

 A few small blood vessels of an essentially capillary nature can 

 now be found in the center as well as in the periphery of the 

 thymic lobules. 



The thymus now contains many lymphocytes. The small 

 lymphocytes (S.L.) are more numerous than in the previous 

 stage. The medium-sized lymphocytes have also greatly in- 

 creased in number while the number of large lymphocytes has 

 remained about the same. Mitoses of both the lymphocytes 

 (M.L.) and the epithelial nuclei (M.c.N.) are of frequent occur- 



