52 On the Origin of the Sertoli or Foot-cells of the Testis. 



common in leucocytes, in bone marrow, and lymphatic glands. While we 

 have occasionally seen somatic division figures among these undifferentiated 

 cells of the embryo testis, this form of reduced division is by far more 

 common. In the male ova themselves at this stage the only mitotic figures 

 we have seen have been typically somatic, and it is hardly possible to 

 confuse the one kind of cell with the other. 



The fact that we have what are apparently second maiotic divisions in 

 cells which are obviously not destined to become sexual cells is in itself 

 very suggestive, but when we come to compare the cells in which this 

 happens with cells that are undoubtedly leucocytic in character, the 

 suggestion is very much strengthened (figs. 10 and 11). We use the term 

 leucocyte in the widest sense, and under it include all the wandering cells 

 of the body. 



Our conclusioii' is, therefore, that the foot-cells of the testis and the cells 

 forming the walls of the tubules or pockets have immediately common 

 ancestors, and that if these cells are not identical with certain stages in the 

 series of leucocytic generations, they are derived from cells that were 

 identical not more than two or three generations before. 



DESCEIPTION OF PLATES. 

 Plate 5. 



Fig. 1. — Early stage in the development of the testis. Signs of the wall of the tubule 

 are visible in places. A division figure among the undifferentiated cells is 

 shown. (Embryo guinea-pig.) 



Fig. 2.— Later stage of the same. The wall of the tubule is formed. 



Fig. 3. — Part of a tubule of adult testis, showing foot-cells. (Guinea-pig.) 



Plate 6. 



Fig. 4.— Early stage in the testis of Triton. The walls of the pockets are not everywhere 

 defined. 



Fig. 5.— The same, showing the undifferentiated cells wandering in among the male ova. 

 Fig. 6. — The same, showing pockets in process of differentiation and one completely 

 formed. 



Fig. 10. — Leucocytes in the lymphatic gland of guinea-pig. 

 Fig. 11. — Leucocytes in the spleen of Triton. 



