192 SERIAL SECTIONS [CH. VII 



With cylindrical objects, especially botanical specimens, one may cut tangen- 

 tial sections, i. e., sections at right angles to a radius, or parallel with the radii 

 (radial sections), or transections, i. e., sections across the long axis. 



§ 303. Arrangement of Serial Sections. — The numerical order may be very 

 conveniently like the words on a printed page, i. e., beginning at the upper left 

 hand corner and extending from left to right. 



The position of the various aspects of the sections should be in general such 

 that when they are under the compound microscope the rights and lefts will corre- 

 spond with those of the observer. This may be accomplished as follows for sec- 

 tions made in the three cardinal sectional planes, Transections, Frontal Sections, 

 Sagittal Sections : 



(C) Transection, i. e., sections across the long axis of the embryo or animal 

 dividing it into equal or unequal cephalic and caudal parts. 



(a) In accordance with the generally approved method of numbering serial 

 parts in anatomy, the most cephalic section should be first (Fig. 162). 



(b ) The caudal aspect of the section should face upward toward the cover- 

 glass, the cephalic aspect being next the slide. 



(c) The dorsal aspect should face toward the upper edge of the slide 

 (Fig. 162). 



This arrangement may be easily accomplished for the transections as follows : 

 Imbed the embryo with the right side down, taking the precautions against letting 

 the embryo rest against the bottom of the box ($ 287). 



(1) Mount the block of paraffin containing the embryo so that the tail end 

 will be next the microtome holder. The head will then be cut first. 



(2) Place in the microtome so that the right side of the embryo will meet the 

 edge of the knife. 



(3) Mount as a printed line and the first or cephalic section will be at the 

 upper left hand corner, and the dorsal aspect of the embryo will be toward the 

 upper edge of the slide. 



Under the microscope the rights and lefts will appear as in the observers own 

 body, also the dorsal and ventral aspects so that he can easily locate parts by 

 comparing them with his own body. 



(B) Frontal Sections, i. e., sections lengthwise of the embryo or animal and 

 from right to left (dextral and sinistral), so that it is divided into equal or unequal 

 dorsal and ventral parts. 



Imbed the embryo with the right side down in the imbedding box as before. 



( 1 ) Mount the paraffin block so that the ventral side of the embryo is next 

 the microtome holder. The dorsal side will then be cut first. 



(2) Let the right side of the embr}-o meet the edge of the knife. 



(3) Mount the first section on the left end of the slide as before and so that 

 the sections will be crosswise on the slide, the tail toward the upper edge. Under 

 the compound microscope the head will appear toward the upper edge and the 

 rights and lefts will be as in the observer's own body. 



(C) Sagittal Sections, that is sections lengthwise of the embrj^o or animal 

 and from the ventral to the dorsal side, thus dividing the body into equal or 

 unequal right and left parts. 



For these sections imbed the embryo as in the two preceding cases. 



