360 METHODS. 



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: 10 p. 



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, 10 //. 



Medium " 15 IJL. 



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, 10 //. 

 Medium " 15 //. 

 Large " 20 p. 



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

 The series is to begin with the ventral side. 



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

 In mounting leave space for the label at the left-hand end of the slide. 

 Keep the sections in the order cut. Arrange them on the slides in the sequence 

 of ordinary written lines. 



Microtomes. 



There are many forms of microtome which may be used with good results 

 and which will work very satisfactorily for making sections of small objects. The 

 cutting of larger objects, such as pig embryos of from 15 to 20 mm., and of pieces 

 of the uterus or other organs, is more difficult, and microtomes which work satis- 

 factorily with small objects often fail to give good even sections of more difficult 

 objects. For embryological work a microtome ought, therefore, to be selected 

 which will give perfectly regular sections in long series of any desired thickness 

 from i up to 25 mikrons. It is also desirable for economy of time to have a mi- 

 crotome which works automatically. , These considerations lead the author to 

 recommend for embryological use especially two forms of microtome made by 

 Messrs. Bausch & Lomb, of Rochester, N. Y., and designated by them as the 

 "precision" and "rotary" microtomes. 



