150 WEIGHT, SIZE, AND AGE OF THE HUMAN EMBRYO. 



moved — that is, the length of the embryo — with an accuracy of 0.05 mm. With 

 this device one gets the same result on repeated trials; moreover, what is more 

 important, different observers get the same result. On account of its accuracy 

 we have found this instrument of value in testing changes in size produced by 

 fixatives and dehydrating solutions. One must take the precaution, however, of 

 always having the embryo placed so that its midsagittal plane is perpendicular to 

 the axis of the observing tube. In other words, the embryo should be in true pro- 

 file. The data included in this paper do not cover very small embryos, where the 

 crown-rump length (center of mid-brain to coccygeal region) is less than the greatest 

 body length. I may note, however, that our rule for such specimens is to use the 

 latter measurement, for the reason that it is subject to less variation due to chance 

 posture of the embryo. 



In addition to sitting height, the head size and foot length were recorded for 

 the purpose of correlation with weight. The head size was recorded in the form of 

 a head modulus, expressed in millimeters and consisting of the mean between the 

 greatest horizontal circumference of the head and the biauricular transverse arc. 

 It was found convenient to measure the latter with a thin strip of paraffined paper 

 placed at the external auditory meatus (in older specimens the tragus) of one side 

 and extending over the apex of the head to the opposite external auditory meatus, 

 the paper then being laid opposite a millimeter rule and the reading taken. Similar 

 paper strips were used for obtaining the head circumference. 



The foot length was found to be of value as a third measurement. This was 

 taken with a small sliding compass, the length being measured from the posterior 

 surface of the heel to the tip of the first or second toe, whichever was the longer. 

 Where there was a difference in the size of the two feet the longer one was recorded. 

 The foot is usually quite straight, but in older specimens it is sometimes flexed, 

 and in such cases it should be held as straight as possible during the measurement. 



AGE. 



For a considerable number of our specimens we have been able to obtain a 

 record of the menstrual age, which I have recorded in the tables. These data are 

 sufficient to construct a satisfactory curve of mean menstrual age for the first 28 

 weeks of pregnancy. For the last 12 weeks resort has been had to data from other 

 sources, thus completing the curve for the whole period of pregnancy; this is repro- 

 duced in chart 6. Based on this curve, the age has been entered for each week 

 along the upper margins of the size- weight curves of charts 1 to 5. It is hoped that 

 this will prove of convenience to other workers who, for clinical purposes, may have 

 occasion to determine the age of fetuses for which no age data are available. By 

 the use of these charts it is possible to estimate the menstrual age of a given fetus, 

 either from weight or from any one of the plotted measurements ; i. e., sitting height, 

 head-size modulus, or foot length, although, where all of these data can be used 

 jointly in placing a specimen, the reading for age is proportionately more accurate 

 by so doing. 



From the group of 1,200 cases gathered by Mall (1918), it was found that the 

 duration of pregnancy, when reckoned from the last menstrual period, was fully 



