170 SERIAL SECTIONS. \_CH. VII. 



(B) Frontal sections, i. e., sections lengthwise of the embryo or ani- 

 mal and from right to left (dextral and sinistral), so that it is divided 

 into equal or unequal dorsal and ventral parts. 



The embryo is so imbedded and arranged in the microtome that the 

 dorsal part is cut first. The first section is then placed in the upper 

 left hand corner (No. i, Fig. 135) dorsal side up, and the cephalic end 

 toward the lower edge of the slide. The microscopic image will then 

 appear with right and left as in the, observer. 



(C) Sagittal sections, i. e., sections lengthwise of the embryo or ani- 

 mal, and from the ventral to the dorsal side, thus dividing it into equal 

 or unequal right and left parts. For these the left side of the embryo is 

 placed up so that it is cut first. The first section is placed in the upper 

 left hand corner of the slide, the left aspect facing away from the slide 

 and the head to the right end, the ventral side toward the upper edge 

 of the slide. Under the microscope the dorsal side will then appear 

 tow T ard the upper edge of the slide and the head to the left. 



§ 288. 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 im- 

 bedded that the cardinal aspects, dextral and sinistral, dorsal and ven- 

 tral, cephalic and caudal, shall be known with certainty. 



UPPER EDGE OF SLIDE. 



Series No. 75. Cover .15 mm. 



Slide No. 1. 



Transections of a Diemyetylus 



Embryo. 



Sections 1-10. 



Total thickness of Sections, 1 mm. 



May 20, 1892. 



Fig. 135. — Labeled Slide of Serial Sections. 



§ 289. Thickness of Cover. Glass and of Serial Sections. — It is 

 a great advantage to use very thin cover-glasses (ywu to 1V0 mm -) for 

 serial sections, then the cover will not prevent the use of high powers. 

 When the ordinary slides (25 X 76 mm., 1 X 3 inch) are used cover- 

 glasses 23 x 55 mm. may be advantageously employed. 



The combined thickness of the sections on a slide is easily determined 

 by noting carefully the position of the microtome screw at the first and 

 last sections, and measuring the elevation. Then if the sections are 



