GERM-CELLS OF PHRYNOSOMA CORNUTUM. 1 23 



embryo ; one is lateral to the prosencephalon, one to the dien- 

 cephalon, and one to the mesencephalon, the fourth being in the 

 region of the spinal cord. Also worthy of note are the two 

 within the cavity of the yolk-stalk in series 12, and a third, in 

 the section preceding, which lies partly in the entoblast, partly in 

 the cavity of the yolk-stalk, into which it is apparently migrating. 



The figures in series 12 have only comparative, not absolute, 

 value, owing to the facts that some sections were folded so that 

 it was impossible to determine whether or not they contained 

 germ-cells, and that I counted only those cells the nature of 

 which is indisputable. An exact count would undoubtedly give 

 a larger total than 98 ; but this, when compared with the total 

 of 1 7 in the younger stage No. 26, requires some explanation of 

 the increase in number. 



Mitoses, although frequent among the somatic cells, were not 

 observed among the germ-cells, indicating the period of rest for 

 the germ-cells observed in other vertebrates, and excluding the 

 explanation of the increase in number of germ-cells by division. 

 Of the three other possible explanations, namely the transforma- 

 tion of somatic cells, or of undifferentiated cells within the 

 embryo, into germ-cells, and the migration of germ-cells from 

 the extra-embryonic region into the embryo, the last seems to be 

 the correct one; because, first, no "transitional cells" were 

 observed, and, second, because germ-cells were found in the 

 blastoderm, and in various positions indicating a migration thence 

 into the embryo. 



Practically every section of the area vasculosa in series No. 26 

 contains germ-cells in the entoblast, cephalad, caudad, and laterad 

 of the embryo. I have not counted them in the entire series, 

 but there are several in each section examined, and must be very 

 numerous. These cells have all the characteristics of germ-cells, 

 the nuclei of the surrounding entoblast, mesoblast, and ectoblast 

 cells being distinctly smaller, as shown in Figs. 3 and 4. The 

 drawings do not show so clearly as I could wish that the germ- 

 cells lie in the entoblast rather than in the mesoblast ; in fact, 

 their size makes the question difficult to determine, especially 

 since entoblast and mesoblast are not very clearly differentiated ; 

 but the nuclei, and often the entire cells, lie below the level of 



