38 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



difficulty of using this lies in the necessity of stopping the action of 

 the hypochlorite before it attacks the ovum. It frees the eggs of 

 the albumen, and does not interfere with staining, but the proportion 

 of eggs destroyed in the process is very great. When the eggs are 

 thrown into weak chromic acid (one fourth to one third per cent) or 

 weak Merkel's fluid, the albumen is coagulated, and if the exposure is 

 properly timed, the egg can sometimes be freed from its envelopes. It 

 is difficult to get good series of sections of eggs hardened in their enve- 

 lopes, or but partially freed from them. The process of dehydrating and 

 embedding renders the albumen so hard and brittle that it breaks into 

 bits when a microtome knife strikes it, and generally the whole section 

 becomes shattered. This is especially true of eggs killed in any of the 

 chromic fluids. The method of Schmidt ('90) for the later stages of 

 Limax was employed with success for the early stages. The eggs are 

 thrown into a saturated aqueous solution of corrosive sublimate, and as 

 soon as they have become opaque they are washed in water, shelled, and 

 freed from the coagulated albumen by a gentle stream of water from a 

 pipette. There is less danger of distorting or destroying the egg in the 

 prucess of shelling by this method than by any other 1 have employed ; 

 the disadvantages are that one is limited to this single killing reagent, 

 and that it is often difficult to remove all the albumen. 



The method which gives by far the best results is as follows. The 

 living eggs are placed in normal salt solution (0.75 per cent), in which 

 they are at once shelled, and then freed from the albumen by washing 

 them in the salt solution, which is dropped upon them from a pipette. 

 The operation is carried on in large glass dishes, resembling watch- 

 glasses, but provided with flat polished bottoms, which are placed upon 

 a black tile ; this renders the eggs visible to the naked eye. The salt 

 solution dissolves away the albumen, leaving the egg entirely free. It 

 can then be transferred to any desired killing reagent by the use of a 

 capillary glass tube. It is advisable to shorten the exposure in the salt 

 solution as much as possible, for nuclear conditions are somewhat altered 

 by its action. Eggs which have lain in it for ten minutes have their 

 nuclear membranes much distended, and the chromatin, gathered into a 

 homogeneous mass at the centre of the nucleus, surrounded by a clear 

 region of nuclear sap. It is possible, however, by this method to obtain 

 eggs whose nuclear conditions do not seem to be in the least changed. 



For killing reagents the following were used : picro-sulphuric ; 

 picro-sulphuric with a few drops of one per cent osmic added to it 

 (Erlanger, '91); Perenyi, followed by five per cent alum water; Whit- 



