626 JOHN BEARD, 



for 24 hours. This is the only satisfactory stain for the yolk-plates. 

 It dyes them a jet-black, and this colour clings to them, even after 

 extraction from cell-nucleus or protoplasm. But it is not a nuclear 

 stain for cells containing yolk-plates, and it has been found useful to 

 stain the whole embryo first of all in borax-carmine or alum-carmine. 

 If this latter be omitted, the sections may be doubly stained with 

 saffranin or eosin. Of course, no new methods are here described, 

 but the application of the above ones to Elasmobranch embryos is 

 probably rarely resorted to. Embryos preserved in picro-sublimate 

 solution furnish exquisitely beautiful preparations with Heiden- 

 hain's stain. 



Although the figures are taken from embryos variously preserved 

 and stained, uniformity has been aimed at in drawing them. Osmic 

 acid may only blacken some, or even none, of the yolk-plates accord- 

 ing to the strength used. None the less, the yolk-plates have in all 

 cases been depicted, as though the embryo had been stained with 

 Heidenhain's haematoxylin. In no sense are the figures diagrammatic. 



It has been attempted to render faithfully all the details of the 

 germ-cells, but the ordinary tissue-cells are given more in outline and 

 solely for topographical purposes. All the figures have been drawn 

 with the Abbe-Zf:iss camera under a Zeiss 2 mm aprochromatic n. 

 ap. 1.30, and the enlargement is either 750 diameters (Figs. 13, 26, 

 27, 51, and 52), or 1500 (all the rest). 



By uniformly using in this way either ocular 6 or ocular 12, com- 

 parison of the various figures is, of course, made easier. 



The figures have all been reduced to two-thirds of their original 

 sizes, and, therefore, the apparent magnifications are 500 and 1000 

 diameters. 



I. The Crerm-Cells in an Embryo of 33 mm 



{Raja batis No. 454). 



In embryos of about this size, or, to put the matter more broadly, 

 in embryos of 28 to 33 mm, the original or primary germ-cells of 

 Raja hatis seem to reach their culmination. It is to be feared, that 

 this statement may cause astonishment; and, therefore, it may be 

 further explained. One of the objects of the present writing is to 

 demonstrate, that in Raja batis during a long period of the develop- 

 ment the germ-cells have no such restricted distribution, as we have 

 hitherto been inclined to suppose. And it will be shown, that during the 

 like time a very large proportion of the germ-cells occupies what can 



