404 SERIAL MICROSCOPIC SECTIONS [Ch. XI 



placed in the box (§ 612) is no hindrance, as the diagram shows 

 through it clearly. 



For embryos and small animals, of which serial sections are to be 

 made, there should always be a photograph natural size. 



The diagram for orientation is easily made from such a photograph 

 by the use of the drawing shelf (fig. 247, A.D.S., § 289, 291). As 

 the embryo or animal is always imbedded with the right side down, 

 left side up, one must be sure to have the diagram in the same posi- 

 tion. This is easily accomplished, as one can draw equally well with 

 the photographic print whichever side is up. That is, if the embryo 

 was photographed left side down, the print should be face down on 

 the drawing shelf to bring the diagram in the imbedding box with 

 the left side up. On the other hand, if the photograph was made 

 with the embryo right side down, then the print should be face up 

 when making the diagram on the bottom of the imbedding box. 



With the definite outline of the embryo or animal on the bottom of 

 the imbedding mass one has a good guide for arranging the object 

 for sectioning any desired plane. 



§ 658. Thickness of serial sections. — The thickness of the sec- 

 tions of a series should be known in all cases; and for modeling it is 

 absolutely necessary (§665, 669). The thickness usually depends 

 somewhat upon the size of the object to be made into series. If the 

 object is small the sections can be thin without having an unmanage- 

 able number of slides. With larger objects - the sections are naturally 

 made thicker to keep the length of the series within bounds. 



One of the following thicknesses will be found to meet nearly all 

 requirements and make modeling easier than as if some odd number 

 of microns were used: $n, 10/z, 15^, 20^, 25/x, 30^1, 40JU, $ofx, 75/z, 100/x. 

 Of course every investigator decides for himself the thickness of sec- 

 tion which will serve his purposes best. 



§ 659. Arrangement of sections on the slide. — (1) A satisfactory 

 and widely adopted method is to arrange the the sections like the 

 printed words in a book. This brings the first section at the upper 

 left-hand corner of the series, and the last section at the lower right- 

 hand corner (fig. 239). 



(2) It is a great advantage to have the sections so arranged on 



