472 



on safer and more certain ground. In every case we find some of 

 Kückert's megaspheres, and these — practically without exception — 

 after very prolonged and careful examination and consideration I 

 identify as germ-cells, or as the immediate fore-runners of such. 



Apart from normally placed germ-cells, a varying number of those 

 megaspheres is found on proper staining in every skate-embryo of 6 

 to 20 mm. They also vary in size from 0,02 to 0,036 up to 0,05 mm. 

 The smaller of them, of which there are many drawings in the plates 

 (figs. 21, 23 to 28, 30, 31) are of exactly the size and appearance 

 of normal germ-cells. The larger ones (figs. 8, 13, 18, 29, 36) only 

 differ from the latter in size. In all other respects — unless they 

 be multinuclear, and, therefore, in degeneration — they are quite like 

 normal germ-cells. And this is true of them, wherever they may be. 

 Their size, however, is always such that one, two, or, at most, three 

 mitoses would bring them down with great accuracy to the average 

 size of an ordinary germ-cell. If size were to be a bar to their germ- 

 nature, it would also prohibit any genetic connection between the first- 

 formed leucocytes and the later and smaller ones, which undoubtedly 

 arise from them. 



And in fact, as a reductio ad absurdum, the first cells of 

 epiblast, mesoblast, and hypoblast could not be connected with the 

 later ones; because, as even the drawings of the present memoir show, 

 they are much larger than the latter. 



Apart from the resemblances between these "megaspheres" and 

 germ-cells, there are also the facts of similar features in their degener- 

 ation. We cannot account for the small number of germ-cells in early 

 embryos, or their absence from the mesentery, where later-on they 

 are so abundant, without the inclusion of the wandering "megaspheres" 

 among the germ-cells. Moreover, there is such an unbroken 

 transition from the ordinary germ -cells of my embryos 

 to the largest of the "megaspheres", and so many of the 

 latter agree so absolutely with ordinary germ-cells in 

 every respect except in. position, that it is quite im- 

 possible to draw any line between them. In the normal 

 position, in the germinal nidus one also encounters abnormally large 

 germ-cells or "megaspheres". 



To sum up: the interpretation of the "megaspheres" as germ- 

 cells, or, if large, as the foreruuuers of such, accounts for them in 

 such a manner as to as to make any further explanation a superfluity. 

 It is required, in order to avoid the insuperable difficulty of attempting 

 to draw a hard and fast line, where such a thing has no shadow of 



