DEPARTMENT OF EMBRYOLOGY. 



George L. Streeter, Director. 



One of the major projects in the program of work of this laboratory is 

 the standardization of stages in the normal development of the human embryo. 

 Our provisional plan is limited to the period beginning with the closure of 

 the anterior neuropore and ending with specimens about 30 mm. long, repre- 

 senting the time from the beginning of the fourth week to the end of the ninth 

 week of development, when the principal features of the external form of 

 the body have become established. We have found that this period can be 

 subdivided into 16 stages which are clearly marked off from one another. 

 Of these stages careful studies of 7 have already been completed. Duplicate 

 models of them, showing the details of the external form of the embryo, have 

 been prepared and placed in a few collaborating embryological laboratories, 

 in order that the proposed subdivisions may be tested out by other workers. 



The adoption of stages of development has been found necessary, as it has 

 become apparent that we must have other objective characteristics than 

 mere size for the determination of age. The practice of basing such deter- 

 mination on the length of the specimen, which is the custom at present 

 among anatomists, has proved unsatisfactory in several respects. A partic- 

 ular source of error lies in the fact that young embryos vary greatly in length, 

 according to the posture in which they happen to undergo fixation. Further- 

 more, when placed in formalin or other fixing solution they become distended 

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

 in volume varies with 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. Moreover, this acquired 

 distention gradually disappears and the size and weight of a given specimen 

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

 sources of inaccuracy, which are disturbing in the case of young embryos, are of 

 less importance in larger fetuses, because in these it is possible to standardize 

 the measurements more accurately and to control fully the posture of the 

 specimen. In the large fetuses, also, the increasing imperviousness of the 

 integument retards the absorption of the fixative solution and the weekly 

 increment in size reaches proportions that render the distention a factor of 

 progressively diminishing importance. For the larger specimens a fairly 

 satisfactory curve of growth, showing the correlation between weight, sitting 

 height, head-size, foot-length, and menstrual age from the eighth week to 

 term, has already been published from this laboratory and was referred to 

 in the Year Book for 1920. 



The period during which length is particularly unreliable as an indication 

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

 development, is the first two months. This was pointed out by Dr. Mall, 

 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 more carefully than had previously been done 

 the details of the external form. This is particularly true of the human 



i Address: Johns Hopkins Medical School, Baltimore, Maryland. 

 76 



