The germ-cells. 625 



perienced embryologist — even though he be a past master of 

 methods — must often feel a desire to know how his brethren have 

 dealt with their material. And in the present instance, because time 

 has been devoted to the discovery of the methods best suited to 

 reveal the germ-cells in Elasmobranchs , some slight service may 

 possibly be rendered to other investigators by recording them. 



To the working embryologist facts are of much more importance 

 than methods; but, if the latter enable another readily to observe 

 the phenomena for himself, their description may not be out of 

 place. 



It is many years (1889) since the writer first began to cultivate 

 and rear skate-embryos. Not in every year has it been possible to 

 get material. In this period, out of all the eggs obtained, a collec- 

 tion of possibly a thousand embryos has been brought together. The 

 help of others in doing this has already been acknowledged elsewhere, 

 but once again I should like to express my indebtedness to my 

 friend, Mr. P. Jamieson, Assistant Naturalist to the Scottish Fishery 

 Board. 



The embryos of the collection have been preserved in many ways. 

 Of these only two are of use in the study of the problems of the 

 germ-cells. For a long period the germ-cells of Raja bafis retain 

 some of their yolk, and with the plates of practically the same sizes 

 as in the beginning. 



The methods found most useful were either those, which tinged 

 yolk-plates (Flemming's osraic mixture), or those, which left them un- 

 altered for subsequent staining (5 '^/o corrosive sublimate with one 

 volume in ten of saturated picric acid). A good modification of the 

 first method was found to be Müller's fluid containing osmic acid. 

 One of the finest embryos of the older collection (No. 454) was pre- 

 pared in this way, and in it the germ-cells stand out with starthng 

 clearness. More recently, and more especially in view of the sub- 

 sequent staining, it has been proved, that 5°/o corrosive sublimate 

 with or without picric acid added, as stated above, furnishes prepara- 

 tions, leaving nothing to be desired, whether they be used for the 

 study of the germ-cells, or of other organs. The sections were laid 

 out on albuminised slides, previously dipped in water. They were 

 then stretched on the water-bath, dried, and afterwards stained. By 

 this method it is possible to re-stain a slide at any future time, if 

 desired, without loss of any of the sections. 



The staining was invariably Heidenhain's iron-alum-haematoxyliu 



