1906.] Origin of the Sertoli or Foot-cells of the Testis. 51 



The processes that are gone through in the testis of Triton seem to offer 

 a further confirmation of the conclusions suggested above. Here the normal 

 course of events in the male ova seems to be : (1) amitosis ; (2) somatic 

 mitoses ; (3) the maiotic phase ; and (4) the conversion of the cells resulting 

 from the second maiotic (homotype) division into spermatozoa. These 

 periods are sharply defined and are easy to follow. While amitosis is 

 taking place, the individual male ova seem to be scattered amongst a number 

 of " undifferentiated " cells, just as happens in the embryo testis of mammals. 

 At this time the future pockets are but ill-defined or not defined at all. 

 Apparently each individual male ovum eventually gives rise to a pocket 

 (fig. 4). A little later on we see that the cells surrounding the male ova 

 wander in between them as they multiply amitotically, until a stage is 

 arrived at when it is impossible to say whether a particular one of them 

 is going to become a cell forming the wall of a pocket or one of the cells 

 which is enclosed within that wall together with the male ova (figs. 5, 6). 

 To the latter the spermatozoa eventually become attached. 



It will thus be seen that what happens in the development of the 

 embryo testis of the mammal is parallel with what happens every year in 

 the testis of Triton. Certain undifferentiated cells which surround the 

 male ova are more or less differentiated along different line"s, some 

 apparently becoming cells to which the spermatozoa are attached during 

 a certain period of time, others forming the walls of the tubules or pockets 

 as the case may be. 



It will be found that there are, among the undifferentiated cells in the 

 early embryo testis of mammals, forms of division where the chromosomes 

 are apparently reduced in number and different in shape to what is seen 

 in ordinary somatic division (figs. 7, 8, and 9). These closely resemble 



Fig. 7. Fig. 8. Fig. 9. 



Mitotic figures in undifferentiated cells surrounding'the male ova. 



what has been described as a second maiotic (homotype) division figure. 

 It has been shown by one of us* that this form of division is very 



* C. E. Walker, " Observations on the Life-History of Leucocytes " (next paper, p. 53). 



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