510 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



solution should be drawn away with blotting-paper so as to bring 

 the blastoderm into close contact with the glass ; take great care 

 that the blotting-paper does not actually touch the blastoderm as in 

 that event it will be apt to stick to it. Now take the coverslip 

 between the finger and thumb and with the aid of a pipette place a 

 very small drop of corrosive sublimate solution (or other fixing fluid) 

 upon the centre of the blastoderm. This will cause the blastoderm 

 to adhere to the coverslip. Now invert the coverslip and drop it on 

 to the surface of some fixing fluid in a watch-glass. 



The blastoderm is then passed through the various operations of 

 staining, dehydrating and clearing, preparatory to mounting whole or 

 conversion into a series of sections as the case may be. The advan- 

 tage of having the blastoderm adherent to a coverslip is that it makes 

 it easier to handle and above all it keeps it from becoming wrinkled 

 or folded. The blastoderm if fixed in corrosive sublimate can usually 

 easily be detached from the coverslip at the stage of clearing if it 

 has not already become free at some preceding stage. Should it 

 adhere obstinately it should be placed in acidulated alcohol for an 

 hour or more. 



The examination of the blastoderm should be carried out as 

 follows : 



1. Study the blastoderm and embryo as a whole under a, prefer- 

 ably binocular, dissecting microscope while it is submerged in the 

 fixing fluid. As the fixing fluid penetrates the embryo the various 

 details in its structure come into view. Continue the examination 

 of the surface relief in the alcohol which is used for getting rid of the 

 excess of the fixing agent. After examining from the dorsal side 

 invert the blastoderm and examine from below. 



2. Repeat the examination of the embryo as a whole as a trans- 

 parent object after staining and clearing. If the individual embryo 

 is to be cut into sections a careful drawing should be made at this 

 stage, the outline being preferably drawn by means of the camera 

 lucida. 



3. Study serial sections cut transversely to the axis of the 

 embryonic body. 



[Sagittal and horizontal sections will also be useful for study 

 after the transverse ones.] 



III. Early Second-Day Blastoderm. — The same method is 

 used as for the 42-hour stage but special care must be taken on 

 account of the more fragile character of the blastoderm. In all 

 probability the blastoderm will remain adherent to the vitelline 

 membrane in spite of repeated shaking and the process of detach- 

 ment will have to be started by gently easing up the edge of the 

 blastoderm on the side next the forceps in which the edge of the 

 circle of vitelline membrane is held. 



To get rid of adherent yolk the circle of vitelline membrane 

 should be laid on the bottom of the dish of salt solution, blastoderm 

 uppermost. A pipette with a wide mouth should be held vertically 



