406 



SERIAL MICROSCOPIC SECTIONS 



[Ch. XI 



(2) If the embryo or animal is of moderate size, that is, not over 

 30 to 35 mm. long, one can use advantageously the intermediate size 

 of slides (fig. 186), that is, those 38 x 75 mm. A suitable cover- 

 glass is 35 x 50 or 35 x 60 mm. 



(3) For objects of considerable size, i.e., over 35 mm. in length, 

 if sagittal or frontal sections are to be made, and if they are to be 

 mounted crosswise, the slide must be of sufficient width. Ordinarily 

 the large standard, 50 x 75 mm., will answer (fig. 187). For the large 

 slides the covers can be 48 x 60 or 48 x 65 mm. For special large 

 sizes of object, special slides can be made of lantern slide covers or 

 old negative glass, etc., and for cover-glasses one can go back to the 

 earlier workers and use mica. 



Do not use too thick cover-glasses or high powers cannot be em- 

 ployed in studying the sections (§ 76-81). 



Transections or Cross Sections 



§ 662. Transections are those made by dividing the body into sec- 

 tions made across the long axis of the body. This divides the embryo 



Imbedded Embryo 



Fig. 238. Serial Transections. 



At the left is the embryo in the imbedding mass and attached to the microtome 

 holder. 



At the right is a glass slide showing how the sections are to be mounted. 



Imbedded embryo It is in the proper position for transections. 



In section 1, the word cephalic shows that the section is cephalic face up; the 

 caudal face rests on the slide. In the middle section the words indicate the edges 

 of the section. Under the microscope the words will be erect. Invert the book 

 and the appearance will be the same as under the microscope. 



or animal into equal or unequal cephalic and caudal segments. With 

 microscopic sections of course the segments of the entire body are 

 very unequal, although each section may be of equal thickness. 



(1) Imbed the embryo or animal with the right side down, taking 

 the precaution to have a layer of partly solidified paraffin at the 



