158 STUDY OF PIG EMBRYOS. 



causes a granular, non-adherent coagulum to form from the foetal fluids. If the 

 specimen is put directly into Zenker's fluid, a fibrous coagulum is formed which 

 often adheres closely to the embryo so as to obscure its shape. Such a fibrous 

 coagulum cannot be removed without injuring the embryo. After having re- 

 mained a proper length of time in the Zenker solution, specimens are further 

 washed for twenty-four hours in running water, and then treated with alcohol 

 and iodine in the usual manner. 



The Making of Serial Sections. 



Specimens should be colored with alum cochineal in toto, then imbedded in 

 paraffin and cut into serial sections according to the directions given in 

 Chapter VIII. 



Selection of the Planes of Section and the Stages for Practical Study. 



It is customary to distinguish three fundamental planes the transverse, the 

 sagittal, and the frontal. It is impossible to so define these planes that the defini- 

 tion shall be exact for all stages. But in general it may be said, the reference 

 being had to the entire embryo, that the transverse plane is one which will be at 

 right angles to the notochord and medullary tube at the level of the heart ; that 

 the frontal plane will be one at right angles to this, passing symmetrically through 

 the limbs of the embryo; and, finally, that the sagittal plane is one parallel to 

 the median plane of the body. As in younger embryos the form is very asym- 

 metrical, both the head and caudal end of the embryo being twisted to one side, 

 the planes which would be true for the body of the embryo in the region of the 

 heart -would not be true elsewhere. For the practical use of the student, there- 

 fore, in these younger stages it is better to determine the direction of the plane 

 by the floor of the fourth ventricle, so that by " transverse" will be understood 

 a plane of section which cuts the head of the embryo symmetrically, no matter 

 how it may cut the body, and which runs parallel to the floor of the fourth ven- 

 tricle (medulla oblongata). The frontal plane should be perpendicular to this 

 and also cut the head of the embryo symmetrically. The sagittal plane in these 

 cases is also that of the head and not of the body. Such planes are recom- 

 mended because in the study of the sections more is gained by having the planes 

 readily understood in the region of the head than in the region of the body. In 

 later stages, when the body has become straighter, the difference in planes for the 

 head and the body may be practically left out of consideration, except that for 

 the heads of older pigs when they are cut alone, as on account of the size of 

 the body is often desirable, the frontal plane is chosen so as to run at right 

 angles to the plane of the palate and symmetrically through the embryo. Sec- 



