388 METHODS. 



It will be found convenient to have a large plate of iron, not less than one eighth of 

 an inch in thickness, which may be placed upon supporters. The tin pan should be 

 set upon this plate and the plate heated by lamps below in order to keep the water 

 hot enough to allow the wax to spread evenly over the surface of the water. The 

 water must be freed from air before the wax is poured in, but must not be allowed to 

 boil after the wax has been added. If bubbles appear in the wax plate, they may be 

 removed while the wax is still hot by directing the blue flame from a Bunsen burner 

 down upon them. If the pan is heated directly without the iron plate, it is sure 

 to warp and become unfit for use. Thin iron plates are also liable to be warped. 

 To determine the thickness of the plates cast as described we proceed em- 

 pirically. A weighed quantity of wax is melted and poured into the pan. After 

 the plate has solidified it is removed by cutting it free from the edges of the pan, 

 and the thickness of the plate is then measured at various points by micrometer 

 callipers. From these data it is easy to calculate exactly what thickness of plate 

 one gram of beeswax represents. To get accurate results it is advisable to cast 

 several plates of varying thickness and determine the average for one gram in 

 that way. Having determined what one gram represents in thickness, it becomes 

 thereafter only necessary to weigh out the proper number of grams in order to 

 obtain any desired thickness of wax plate. It will be found advantageous to filter 

 the wax before using it. This may easily be done by a double hot-water filter. 

 Such a filter may be made of copper. It is desirable to connect it with a Mariotti's 

 flask to maintain a constant water level. 



Directions for Orienting Serial Sections of Embryos. (NOTE: The lower 

 edge of the ribbon is the one to the left, when the observer has the object between 

 himself and the knife.) 



1. Transverse Series. 



Normal thickness: io//. 



Dorsal surface to be toward the lower edge of the ribbon. 

 Series to begin with the head. 

 In cutting, the left side of the embryo must strike the knife first. 



2. Sagittal Series. 



Normal thickness: Small embryos, io/*. 

 Medium " i5//. 

 Large 20^. 



The head of the embryo to be toward the lower edge of the ribbon. 

 Series to begin with the right side. 



In cutting, the ventral side of the embryo must strike the knife first. 

 ^3. Frontal Sections. 



Normal thickness: Small embryos, iofi. 

 Medium " 15;*. 

 Large 20;*. 



