4 EARLY EMBRYOLOGY OF THE CHICK 



appreciate the complete relationships of a structure. The 

 nature of embryological material, however, introduces new- 

 problems. Embryos of the age when the establishment of the 

 various organ systems and processes of body formation are being 

 initiated are too small to admit of successful dissection, but 

 not sufficiently small to permit of the satisfactory micro- 

 scopical study of an entire embryo, except for its more general 

 organization. To study embryos of this stage with any degree 

 of thoroughness they must be cut into sections which are 

 sufficiently thin to allow effective use of the microscope to 

 ascertain cellular organization and detailed structural relation- 

 ships. In preparing such material the entire embryo is cut into 

 sections which are mounted on slides in the order in which 

 they were cut. A sectional view of any region of the embryo 

 is then available for study. 



While sections readily yield accurate information about local 

 regions it is extremely difficult to construct a mental picture 

 of any whole organism from a study of serial sections alone. 

 For this reason it is necessary to work first on entire embryos 

 which have been prepared by staining and clearing so they may 

 be studied as transparent objects. From such preparations 

 it is possible to map out the configuration of the body, and the 

 location and extent of the more conspicuous internal organs. 

 In this work the fact that embryos have three dimensions must 

 be kept constantly in mind and the depth at which a structure 

 lies must be determined as well as its apparent position in 

 surface view. While conventionally entire chick embryos are 

 represented in dorsal view, much additional information may be 

 gained by following a study of the dorsal, with a study of the 

 ventral aspect. Unless the preliminary study of entire embryos 

 is carefully and thoroughly carried out the study of sections 

 will yield only confusion. 



In studying a section of an embryo it is necessary first of all 

 to determine its location. The plane of the section under 

 consideration, and the region of the embryo through which it 

 passes should be ascertained by comparing it with an entire 

 embryo of the same age as that from which the section was cut. 

 Only when the exact location of a section is known can the 

 structures appearing in it be correlated with the organization of 

 the embryo as a whole. Probably nothing in the study of 



