322 SERIAL SECTIONS OF EMBR YOS [ CH. X 



482. 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) Put the right side of the embryo next the microtome 

 holder, then the left side is cut first. 



(2) L,et 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 brings the ventral side up and the head to the right on 

 the slide. Under the microscope the head appears at the left and 

 the dorsal side away from the observer (Fig. 253). 



(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). 



483. Axes for Sections. For transections cut across the 

 longest straight line from head to tail. 



For sagittal sections select the straightest embryo and cut par- 

 allel with the longest axis dorso- ventral. 



For frontal sections cut parallel with the long axis, dextro- 

 sinistral. 



484. For serial sections with collodion imbedded objects it 

 is a great advantage 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. 



485. Thickness of Cover-Glass for Serial Sections. 

 It is a great advantage 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 (25X76 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 multiplying the number of sections by the thickness 

 of each. 



