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MAY M. lARVIS. 



the blood-vessels, and the lower wall of the cells, where distin- 

 guishable, touches the boundary of the entoblast, so that I con- 

 clude that they lie normally in the entoblast. 



No germ-cells were observed in the extra-embryonic blasto- 

 derm of No. 12 ; in series No. 15, an embryo of the same age, 

 of which about one half the sections were examined in detail, one 

 germ-cell was found in the blastoderm ; they were present in the 

 blastoderm of series No. 27. 



The path of normal migration is the entoblast of blastoderm, 

 yolk-stalk, and intestine, and the sclerotome of the mesentery to 

 the germinal anlagen ; the only other possible path is the meso- 

 blast of the same structures, and the following table shows that 

 the first is the normal path : 



Those cells that " lose the way" are found in the various po- 

 sitions tabulated above. It will be observed that the migration 

 is just beginning in series 26, and practically completed in No. 1 2. 



This controverts the suggestion that the cells are passively 

 carried by the concrescence of the germ-layers, since this growth 

 is too slight, between the stages studied, to account for the trans- 

 position of the germ-cells. 



Now the total number of germ-cells in the older embryo, even 

 allowing for the folded sections, is not nearly equal to the total 

 number of extra-embryonic germ-cells in the younger embryo. 

 The most obvious explanation of the deficit is Beard's theory of 

 degeneration. My observations appear to uphold Beard ; I find 

 cells resembling germ-cells in every respect except the size of 

 the nuclei, these being about equal to the somatic nuclei, or ap- 

 parently absent entirely. They are especially numerous in the 

 sclerotome around the aorta, where the abnormally placed cells 

 are also most numerous. They occur in the cavity of the yolk- 

 stalk, and apparently in the intestinal lumen ; the presence of 



