THE STUDY OF THE EXTERNAL FORM. 159 



tions through the head at right angles to this may be designated as horizontal.* 

 Students will find that it is very much easier to study transverse and frontal sec- 

 tions when they are symmetrical. No pains, therefore, should be spared to 

 orient the embryo properly in the microtome before the sections are cut. 



Selection of the Stages. The most profitable stage to study is that of an 

 embryo of from 9 to 13 mm. in length. Each student should have three speci- 

 mens of this stage, and it is advantageous that the specimens given each student 

 be approximately of the same size. The embryos should be first studied care- 

 fully as to their external form and then cut into serial sections in the transverse, 

 sagittal, and frontal planes. Of these, the transverse series should form the 

 principal basis of study, and the other series should be used principally to clear 

 up the student's conception of the relation of parts. Embryo pigs of the size 

 specified have the typical class characteristics of mammalian embryos, and may 

 readily be distinguished from the embryos of any other class of vertebrates. The 

 differentiation of the anlages of all the important organs is accomplished, so that 

 these anlages can be identified with certainty and their genetic relations to the 

 adult structures can be clearly grasped by the student. At the same time, 

 although the anatomical differentiation is well advanced, the histological differ- 

 entiation has made very little progress, so that the stages in question are par- 

 ticularly instructive to beginners. The anatomy of the pig at this stage is, 

 therefore, readily understood by the student who knows the general anatomy of 

 the adult. Older embryos are more complicated and yield such long series of 

 sections that the beginner is apt to be discouraged. Younger embryos, owing 

 to their spiral twisting, are exceedingly difficult for students to understand when 

 sectioned. After having thoroughly mastered the structure of the pig embryo 

 of from 9 to 13 mm., the student may advantageously extend his study of em- 

 bryos to other sizes. If, as is done in this work, the principal study is made with 

 embryos of 12 mm., the student may proceed to make sections of other stages as 

 follows : 



Pig embryo of 9 mm. , transverse and sagittal series. 



" " " 6 " transverse series. 



" " "17 " transverse series. 



" " " 20 " transverse and sagittal series. 



(Of the head alone, the frontal series.) 

 Pig embryo of 24 mm., of the head alone, frontal series. 



The Study of the External Form. 



The student should make a careful and thorough study of the external form 



*The system of planes here described is that adopted for the Harvard Embryological Collection, and has 

 been found convenient in practice. 



