app. METHODS OF EMBRYOLOGICAL EESEAECH 577 



three changes each of 90% alcohol, absolute alcohol, and xylol or other 

 clearing fluid. The actual process of infiltration with paraffin should last 

 for the minimum time (which will have to be determined by experiment J ) 

 and be carried out at the minimum temperature. 



It may be remembered that the complicated and bulky water-bath 

 with its thermostat is in no way necessary for the embedding process. 

 A very simple apparatus which is perfectly efficient consists of a small 

 metal trough (copper, or tinplate) resting upon a metal table kept 

 heated at one end by a small flame. By sliding the trough lengthwise 

 along the table a position can be found such that the entire thickness of 

 paraffin is fluid at the end next the flame and solid towards the other end. 

 Between these two points stretches an inclined plane of solid paraffin upon 

 the surface of which the embryo rests without any risk of the temperature 

 rising appreciably above melting-point. A simple embedding trough of 

 the kind indicated is of great use in the field as there is no method of 

 storing and transporting embryos so free from danger of accident or of 

 histological deterioration as having them embedded in solid paraffin. 



To get a block of paraffin in good condition for section-cutting the 

 embryo should be transferred to a bath of fresh paraffin as soon as it is 

 infiltrated. With certain clearing agents, e.g. cedar oil, it is well to give 

 two or three changes of paraffin. The vessel containing the embryo in a 

 considerable volume of paraffin should now be floated on cold water so as 

 to give a homogeneous translucent block of solid paraffin. On no account 

 should the vessel be actually submerged in the cold water for in this event 

 the contraction of the inner paraffin as it cools within the already rigid 

 outer layers will lead to the formation of cavities into which the water 

 penetrates. 



For the actual process of section-cutting it is necessary to use a 

 mechanical microtome. The Cambridge Booking microtome is one of the 

 most convenient for ordinary embryological work while the Beinhold- 

 Giltay microtome is a most excellent instrument both as regards accuracy 

 and rapidity of working. 



The paraffin block containing the embryo is trimmed down so as to be 

 rectangular in section and is then fixed by the interposition of a hot 

 spatula to the paraffined surface of the microtome carrier in such a position 

 as may be necessary to give the required direction of sections. 



Where the object is a "difficult" one, e.g. containing much yolk, it is 

 advisable to have it surrounded by a paraffin block of considerable size. 

 A considerable mass of paraffin above the specimen makes it cut better, 

 while a considerable mass to the side causes successive sections, with their 

 long edges, to adhere better together and form a continuous ribbon. The 

 embryo should be near one of the lower corners of the block to facilitate 

 exact orientation. 



For thorough investigation of the structure of embryos it is advisable 

 to have specimens cut into sections in the three sets of planes — transverse, 

 sagittal or longitudinal vertical, and coronal or longitudinal horizontal. 

 To obtain these it is necessary to have the embryo orientated exactly on the 

 microtome. In most cases this can be accomplished with a sufficiently 

 close approximation to accuracy when fixing the paraffin block on to the 



1 E.q. for a Chick at about the middle of the second day about 20 minutes will be 

 found to be sufficient. 



VOL. II 2 P 



