654 JOHN BEARD, 



the epiblast ep^ the open medullary plate w. s, the notochord w, parts 

 of two mesoblastic somites p. v, the unsegmented mesoblast me, which 

 is not yet split into soraatopleure and splanchnopleure, the definitive 

 hypoblast %, and beneath this the yolk-hypoblast y.hy^ in and below 

 which the cells depicted as black circles mainly lie. 



Of the cells in question a small and representative selection has 

 been drawn under the 2 mm apochromatic objective. From motives 

 of economy they are not figured. There are 6 of these cells, labelled 

 in the text-figure A Ato F. These 6 cells are of the following sizes : 

 A— I) = 0.036 mm, E waà F = 0.02 mm. 



Comparison of these with any of the other drawings of normal 

 primary germ-cells of later periods reveals the identity of the two. 

 In other words, the cells labelled J, to i^ in text-figure A, as well as 

 those not lettered in the same figure, all possess the characters of 

 normal germ-cells of about 0.02 mm or of the forerunners of such 

 (0,036 mm). These are their average sizes, their supply of yolk is 

 that characteristic of normal germ-cells, and their cytoplasm has 

 the like glassy appearance. The little group of cells here described 

 is a small portion of the mass already referred to as extending to 

 the front end of the blastoderm. Of the cells lettered one, A in the 

 diagram, lies well within the embryo and, moreover, on the site of 

 the future germinal nidus. Its size, position, and characters are those 

 of a normal germ-cell. Obviously, it has migrated to its present 

 position along the space between the hypoblast and the unsegmented 

 mesoblast. It may be concluded, therefore, that at a somewhat earlier 

 epoch the cell A lay somewhere alongside its fellows of like characters, 

 the other cells B, C, D, E, and F, and the remaining unlettered 

 ones, beneath the embryonic foundation in the upper part of the 

 blastoderm. Some of the other cells are obviously engaged in migrat- 

 ing upwards through the yolk-hypoblast towards the embryo. That 

 labelled C has done this, and now lies between epiblast and hypoblast 

 beyond, that is laterad of, the region of the embryo. As later 

 phases will sufficiently reveal, C also is engaged in wandering 

 into the embryo. Without doubt all these cells are germ- cells, of 

 which only one has yet reached the termination of the germinal path. 



Further back in other sections of the same row there is one such 

 cell applied to the under side of the epiblast of the embryo. Beyond 

 this the next one, which lies at all within the embryonic foundation, 

 is at the very commencement of the germinal path, and following 

 this another one is seen in the hypoblast of the definitive gut to the 



