DEVELOPMENT OF THE AURICLE IN THE HUMAN EMBRYO. 



INTRODUCTION. 



In order to obtain objective criteria for the determination of the age of human 

 embryos, it has been found necessary to seek characteristics other than size. The 

 practice of basing such determinations on the length of the specimen, which is the 

 custom at the present time among anatomists, has proved, in several respects un- 

 satisfactory, particularly as young embryos vary greatly in length according to the 

 posture in which they happen to undergo fixation, and for the further reason that, 

 when placed in formalin or other fixing solution, embryos become distended by 

 the solution to a degree that adds considerably to their length and weight. This 

 increase in volume varies according to the size of the specimen and the condition 

 of its tissues. Smaller specimens undergo a greater relative increase than the 

 larger ones and fresh specimens greater than macerated ones. Furthermore, 

 this acquired distention gradually disappears and hence the size or weight of a 

 given specimen will vary according to the time that has elapsed since its fixation. 



These sources of inaccuracy, which are of disturbing importance in the case 

 of young embryos, are of less importance in larger fetuses, because in these it is 

 possible to standardize more accurately the measurements and to control fully the 

 posture of the specimen. Also, in large fetuses the factor of distention by the 

 fixative is of less moment ; the increasing imperviousness of the integument retards 

 the absorption of the fixative solution and the weekly increment in size reaches 

 proportions that render the fixative distention a factor of progressively diminishing 

 importance. 



The period during which length is particularly unreliable as an indication of 

 the age of a specimen, and for which we are in the greatest need of more accurate 

 criteria of development, is the first two months; that is, from the earliest stages up 

 to about 30 mm. length. This was pointed out by Mall (1914), who proposed the 

 subdivision of this period into stages, based upon the development of external 

 features, such as the branchial arches, arms, and legs. 



In attempting this standardization it soon became apparent that it would 

 be necessary to survey the details of the external form more carefully than had 

 previously been done. This meant the study of more specimens and better photo- 

 graphic records, so planned as best to display individual regions. This is particu- 

 larly true of the human embryo, where the difficulty of distinguishing between 

 real and accidental differences is increased by the varied conditions under which 

 the material for study is obtained. It was, in fact, the recognition of such a need 

 that led Spaulding (1921) to make a detailed study of the steps in the differentiation 

 of the external genitalia. The successful outcome of his investigation testifies to his 

 wisdom in limiting his attention to a definite region. It is clear that before a satis- 

 factory series of developmental stages, based on external form, can be arrived at, 



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