398 CHARLES H. SWIFT 



begins in the male chick embryo at 13 days as a result of pri- 

 mordial germ-cell division, and that the peculiar arrangement 

 of mitochondria around the attraction-sphere, which will be 

 taken up at greater length in a later chapter, distinguishes the 

 spermatogonium from the primordial germ-cell. 



At this time, also, it is necessary to say something about the 

 continuity of the mitochondria in the germ line. 



In this regard, I can say that they are always present in the 

 primordial genn-cells, oogonia and spermatogonia, either as 

 short rods or granules, depending somewhat upon the fixation. 

 About the same amount seems to be present in the germ-cells 

 at all stages, from origin in the primitive streak stage to oogonia 

 and spermatogonia formation in the 8 and 13 day embryo 

 respectively. Even in the oogonia and spermatogonia the num- 

 ber of mitochondria remains about as in the germ-cells, for the 

 mass around the attraction-sphere is formed at the expense of 

 the rest of the cytoplasm (figs. 4, 5 and 6). 



In the 15 day chick embryo there are an immense number of 

 interstitial cells in the stroma. They appear grouped in masses 

 and cords in the stroma between the seminiferous cords. So 

 numerous are they that it is possible to identify them in the 

 section before removal of the paraffin. This can be easily done 

 since they appear black, owing to the staining of their many 

 fatty granules by the osmic acid. 



The seminiferous cords branch and anastomose to form a net 

 as described under the 13 day embryo. They contain, however, 

 relatively and absolutely, many more spermatogonia, for the 

 increase has been in that element owing to continued division 

 of the germ-cells (fig. 3). The number of spermatogonia, com- 

 pared with the preceding stage, is enormous. They are seen 

 several hundred in a field, when the low power microscope is 

 used. Numbers of mitoses in the primordial germ-cells and 

 spermatogonia are present (fig. 3), and this is the stage in which 

 they are found dividing most actively. 



In regard to the 17 day chick embryo not a great deal need 

 be said, for, in it, the findings closely resemble those described 

 in the 15 day embryo. 



