CH.X-] SERIAL SECTIONS OF EMBRYOS 321 



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: 

 5yu, lOyU, 15/*, 20/<, 30/*, 40/*, 50/*, 6o/t, 80/*. 



§ 480. Transections, that is sections across the long axis of 

 the embryo or animal. 



Imbed the embryo with the right side down, taking the pre- 

 caution to have a layer of paraffin between the embryo and bottom 

 of the box (§ 441). 



(1) Mount the block of paraffin containing the embryo so that 

 the tail end is next the microtome holder. The head is then cut 

 first. 



(2) Place in the microtome so that the right side of the embryo 

 meets the edge of the knife. 



(3) Mount as a printed line and the first or cephalic section is 

 at the upper left hand corner, and the dorsal aspect of the embryo 

 is toward the upper edge of the slide. 



Under the microscope the rights and lefts appear as in the observ- 

 er' s own body, also the dorsal and ventral aspects so that he can 

 easily locate parts by comparing them with his own body. 



§ 481. Frontal Sections, that is sections lengthwise of the 

 embryo or animal and from right to left (dextral and sinistral), so 

 that the smbryo 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 is then 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 slides as 

 before so that the sections are crosswise on the slides, the tail toward 

 the upper edge. Under the compound microscope the head appears 

 toward the upper edge and the rights and lefts are 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. 



