INTRODUCTION 5 



embryology causes students more difficulties than neglect to 

 locate sections accurately with the consequent failure to ap- 

 preciate the relationships of the structures seen in them. Too 

 great emphasis cannot be laid on the vital importance of fitting 

 the structures shown by sections properly into the general 

 scheme of organization as it appears in whole-mounts. It 

 must by no means be inferred that the possibilities of the whole- 

 mounts have been exhausted by the preliminary study accorded 

 them before taking up the work on sections. Further and more 

 careful study of entire embryos should constantly accompany 

 the study of serial sections. Many details which in the initial 

 observation of the whole-mount were inconspicuous or abstruse 

 will become significant in the light of the more exact information 

 yielded by the sections. 



In the discussion of structures and processes in embryology, 

 it is necessary to use terms designating location and direction 

 which are referable to the body of the embryo regardless of the 

 position it occupies. The ordinary terms of location, which are 

 primarily referred to the direction of the action of gravity, 

 such as above, over, under etc. are not sufficiently accurate. 

 In gross human anatomy, there still persist many terms that 

 are referred to gravity, and are therefore, because of the erect 

 posture of man, not applicable to comparative anatomy or to 

 embryology. The most confusing of these are anterior and 

 posterior as used in gross human anatomy to mean, respec- 

 tively, pertaining to the belly and to the back. In comparative 

 anatomy and in embryology, anterior has reference to the head 

 region and posterior to the tail region. The use of these terms 

 in embryology in the sense usual in gross human anatomy 

 is likely to lead to confusion and is entirely avoided in this 

 book. The terms anterior and posterior have been replaced 

 to. a large extent by their less confusing synonyms, cephalic 

 and caudal. 



In addition to the adjectives of position, such as dorsal, 

 ventral, cephalic, caudal, mesial, lateral, proximal, distal, 

 corresponding adverbs of motion or direction are commonly 

 used in embryology. These adverbs are formed by adding the 

 sufhx -ad to the root of the adjective, as dorsad meaning toward 

 the back, cephalad meaning toward the head, etc. These 

 must not be used as adjectives of position but should be ap- 



