ON EGGS OF MARINE ANIMALS 67 



perfect clearing of the eggs. From this they are removed to 

 melted paraffin for the imbedding. 



Imbedding 



From the second toluene or xylene the eggs are removed by 

 a pipette thoroughly dried by bringing it into a flame and 

 expelling the air from the bulb. Xylene or toluene is then 

 sucked up into the pipette, care being exercised that the oil 

 does not get into the rubber bulb. The drop of eggs is now 

 pipetted into the receptacle to be used for imbedding. In 

 carrying over the drop of eggs to the receptacle there will be some 

 xylene or toluene' present. This is no serious disadvantage, 

 unless it takes up moisture from the atmosphere. To avoid 

 excessive evaporation of the xylene or toluene brought over with 

 the eggs, the receptacle may be covered with a thin glass bowl 

 from which the water vapor has been driven oflF previously. No 

 eggs should remain in the tube from which the drop has been 

 transported, especially in those cases in which only a few eggs 

 have been carried through. Also, no eggs should stick to the 

 pipette used in making the transfer. The danger of loss of 

 eggs in this wise may be lessened if the pipettes used are scrupu- 

 lously clean and perfectly dry, and if in addition before being 

 used for transferring the eggs they are kept in xylene absolutely 

 free from any trace of water. The slightest trace of water will 

 tend to make the eggs stick to glass. 



For imbedding minute eggs workers employ various recep- 

 tacles and devices. For example, some embryologists imbed 

 the eggs in glass vials which they break after the paraifin has 

 hardened — a miserable method. Still another method is to 

 imbed the eggs in a dish which has a trough in the bottom into 

 which the eggs settle. Other workers use gelatin capsules. Or 

 they imbed the eggs in a packet of frog's epithelium. The dis- 

 advantage of these three latter methods appears when one cuts 

 the paraffin blocks: the sections often do not ribbon properly. 

 In addition, it is a nuisance to trace continuously all the slices 

 of one egg from one row of sections to another especially where 

 many eggs are present. As often is the case, the simplest method 

 is the best. This is to use watch crystals with flat bottoms of 

 about 4 centimeters diameter having a capacity of 10 cc. Or 



