304 EMBRYOLOGICAL METHODS. 



Lay one of the parts of the plate on the outline drawing so that the cut 

 edge covers the line that corresponds to the first section you are going to 

 fill in ; then lay the other part of the plate on the drawing of the section in 

 such a position that the limits of the drawing correspond to the same coloured 

 lines that cover the limits of the outline drawing on the other part of the 

 plate already placed. 



Trace on the plate that covers the drawing oE the section the outline of 

 the internal organs. Lay it against its fellow-plate on the outline drawing, 

 making the coloured lines correspond, and you will easily be able to mark 

 off accurately on the outline drawing a series of dots that correspond in 

 position to the outlines of the internal organs. This operation having been 

 repeated for each of the sections that you desire to bring into your recon- 

 struction, nothing remains but to join your dots by lines, and you will have 

 filled up your outline drawing with a representation of the internal organs 

 in the same plane. 



If any reader think this process complicated, he needs but to spend five 

 minutes in trying it with a piece of tracing paper, and will find it to be in 

 reality extremely simple. 



Another simple plan is to gum the drawings of the sections on cardboard 

 of a thickness proportional to the thickness of the section and the magni- 

 fication, cut out all the cavities o the drawing with a knife or fret-saw, 

 and gum all the fretwork thus obtained together. This gives, of course, a 

 model of the object. 



In simple cases it may be sufficient to adopt the plan 

 described by SCHAFFER (Zeit,f. wiss. Mik., vii, 3, 1890, p. 342). 

 Careful outlines of the sections to be reconstructed are drawn 

 on tracing-paper with the aid of the camera lucida, super- 

 posed, and held up against the light for examination by 

 transparence. EYCLESHEIHER'S method of producing orienta- 

 tion lines in celloidin imbedding mass has been described in 

 Chap. XYI. 



Mammalia. 



588. Babbit. The rabbit may conveniently be taken as a 

 type for this kind of work. 



Dissection. For the study of the early stages the ova must 

 be sought for in the tubse a certain number of hours after 

 copulation. The dehiscence of the follicles takes place about 

 ten hours after the first coitus. The tub SB and cornua having 

 been dissected out should be allowed to cool, and remain until 

 the muscular contractions have ceased. Then, with the aid 

 of fine scissors or a good scalpel, all the folds of the genital 

 duct are carefully freed from their peritoneal investment. 



The tubae are then (if the ova are still within them, which 



