240 PRACTICAL DIRECTIONS. [APP. 



gas-jet. If the vessel be filled with hot water, and allowed to 

 cool down to 40 or thereabouts, before the eggs are intro- 

 duced, a very small gas flame will be sufficient to maintain 

 the requisite temperature. A small pin-hole-nozzle, giving 

 with ordinary pressure an exceedingly narrow jet of flame 

 about two inches high, is the most convenient. By turning 

 the gas off or on, so as to reduce or increase the height of the 

 jet as required, a very steady mean temperature may be 

 maintained. A simple arrangement of this kind on the whole 

 works more satisfactorily than any of the complicated instru- 

 ments which have been introduced for similar purposes. 



In the absence of gas, a patent night-light placed at a 

 proper distance below the bath may be made to answer very 

 well. When a body of water, once raised to the necessary tem- 

 perature, is thoroughly surrounded with non-conducting ma- 

 terial, a very slight constant amount of heat will supply all 

 the loss. 



The temperature should be from 37 to 40C. ; it may rise 

 temporarily a few degrees above 40 without any permanent 

 harm, and should not be allowed to fall below 37. 



The eggs within the bath should be placed on and covered 

 up with cotton wool ; and the products of the combustion of 

 the gas should be kept as much from them as possible. 



II. Examination of a 36 to 48 hours embryo. 



The student will find it by far the best plan to begin 

 with the study of an embryo of this date. The manipulation 

 is not difficult ; and the details of structure are sufficiently 

 simple to allow them to be readily grasped. Earlier embryos 

 are troublesome to manage until some experience has been 

 gained ; and the details of later ones are so many as to render 

 it undesirable to begin with them. 



A. Opening the egg. 



Take the egg warm from the hen or the incubator, 

 and place it (it does not matter in what position, 

 since the blastoderm will at this stage always be found 

 at the uppermost part of the egg) in a small basin 

 large enough to allow the egg to be covered with 

 fluid. It is of advantage, but not necessary, to place 



