328 A. RICHARDS AND JAMES T. THOMPSON. 



plasm. In older embryos these cells may be recognized by their 

 size, since they are larger than any others which may occur in the 

 same region. 



Figures i, 2, 3 and 3a are surface views of typical sex-cells. 

 The peripheral arrangement of the chromatin has been empha- 

 sized in drawing Fig. <\b, by focusing upon a level with the center 

 of the nucleus. Fig. 40 was obtained by focusing higher on the 

 surface of the same nucleus. Thus Fig. 46 represents the chro- 

 matin knots in an optical section ; w r hile Fig. 40 shows them in a 

 surface view. If Fig. 40 were superimposed upon Fig. 4^ the 

 resulting composite \vould be a cell not unlike that represented in 

 Fig. 3, except that in the latter the knots have taken the familiar 

 granular appearance. 



A positive identification of the primary sex-cells was first made 

 in a 24-day embryo. From this stage their path was followed 

 backwards, through all the intermediate phases of migration, 

 until they were no longer evident. It is considered expedient to 

 describe their position in the 24-day stage, so that no question 

 shall arise later as to the exact nature of the migrating cells 

 whose course is to be traced. 



Having established the identification of the sex-cells in the late 

 embryo (24 days) their migration may be traced from their 

 earliest appearance up to this stage. Although this sequence is 

 contrary to our experimental procedure, it is believed to be more 

 easily followed by the reader. 



zj-Day Embryo. 5.75 Mm. Long, At the 24-day stage the 

 sex-cells lie in the sac-like anlagen of the germ glands, which 

 have formed dorsally and slightly laterally to the hind gut. Here 

 they are unquestionably recognizable (Fig. 5). These cells are 

 numerically inferior to the peritoneal cells which surround them 

 and which are beginning to take a very active part in the forma- 

 tion of the future sex gland. The size and position of the germ 

 gland anlagen in relation to the embryo is shown in Figs. 6 and 7. 

 No attempt has been made to ascertain the average number of 

 sex-cells which are present during this stage. 



Whether these are the true sex-cells as maintained by many 

 investigators, or whether they later disintegrate and become re- 

 placed by "secondary genital cells" as indicated by Firket (1914, 



