IQ2 SERIAL SECTIONS [CH. VII 



$ 302. Arrangement of Tissues for Sections in Histology. The}- should be so 

 arranged that the exact relations of each part to the organ can be readily deter- 

 mined. For example, an organ like the intestine, a muscle or a nerve, should be 

 so arranged that exact transections or longisections can be made. Organs like the 

 liver and other glands, the skin, etc., should be so arranged that sections parallel 

 with the surface or at right angles to it, ( surface or vertical sections) may be 

 made. Oblique sections are often very puzzling. 



303. Arrangement of Serial Sections. The numerical order may very con 

 veniently be 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. 



\ 303 a. Imbedding and Sectioning Embryos and Minute Animals. Serial 

 sections of these should be made in the three cardinal sectional planes, viz ; 

 Transections ; Frontal Sections ; Sagittal Sections. 



If models are to be constructed from the sections it may be more conveniently 

 done if the sections are one of the following thicknesses : 5//. lo/w, i5//, 2ojii, 3O//. 

 4oju, 5ow, 6o//, 8o//. With an adjustable wax-plate tablet any thickness of sections 

 may be modeled ($ 437). 



(A) Transections, that is sections across the long axis of the embryo or 

 animal. 



Imbed the embryo with the right side down, taking the precautions to have 

 a layer of paraffin between the embryo and 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 observer's 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, that is sections lengthwise of the embryo or animal 

 and from right to left (dextral and sinistral), so that the embryo 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 embryo 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 



