CH. VI 7 '] SERIAL SECTIONS 193 



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. 



(4) If the sections are too long to mount crosswise of the slide, cut the 

 sections apart and mount with the head to the right. 



(C) Sagittal Sections, that is sections lengthwise of the embryo 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 with the right side down as before. 



1 i ) Put the right side of the embryo next the microtome holder, then the 

 left side will be cut first. 



(2) Let the caudal end meet the knife edge if the embryo is small. 



(3) Put the first section in the upper left hand part of the slide as in the 

 other cases. The sections will be lengthwise of the slide. This will bring the 

 ventral side up and the head to the right on the slide. Under the microscope the 

 head will appear at the left and the dorsal side away from the observer. 



(4) For large or long embryos place the right side next the microtome holder 

 as above, but let either dorsal or ventral aspect meet the knife. Cut the sections 

 apart and mount as in (3). 



(D) Axes for Sections. For transections cut across the longest straight line 

 from head to tail. 



For sagittal sections select the straightest embryo and cut parallel with the 

 longest axis. 



For frontal sections cut parallel with the long axis. 



$ 304. For serial sections with collodion imbedded objects it is a great advan- 

 tage to have the imbedding mass unsymmetrically trimmed, so that if a section is 

 accidentally turned over it may be easily noticed and rectified. 



Furthermore it is imperatively necessary that the object be so imbedded that 

 the cardinal aspects, dextral and sinistral, dorsal and ventral, cephalic and caudal, 

 shall be known with certainty. 



| 305. Thickness of Cover-Glass and of Serial Sections. It is a great advan- 

 tage to use very thin cover-glasses (0.12-0.18 mm.) for serial sections, then the 

 cover will not prevent the use of high powers. When the ordinary slides (25 X 

 76 mm., 1X3 inch) are used, cover-glasses 24 X 50 mm. may be advantageously 

 employed. 



The combined thickness of the sections on a slide is easily determined by not- 

 ing carefully the position of the microtome screw at the first and last sections and 

 measuring the elevation. With good modern automatic microtomes the successive 

 sections are almost exactly uniform in thickness, hence it is easy to determine the 

 combined thickness of the sections on a slide by multiplying the number of sec- 

 tions by the thickness of each. 



\ 306. Labeling Serial Sections. The label of a slide on which serial sections 

 are mounted should contain at least the following : 



