158 



WEIGHT, SIZE, AND AGE OF THE HUMAN EMBRYO. 



the other external auditory meatus). The sum of these, divided by 2, is taken as a 

 head modulus and expressed in millimeters. These measurements were selected 

 because they can be systematically and accurately made for specimens from 50 

 mm. long to term, and because they tend to correct each other when the head is 

 molded, as is frequently the case. If I were going to make these measurements 

 again I should add the biauricular diameter to the transverse arc, thus obtaining 

 a transverse circumference of the head. The average between this and the great- 

 est horizontal circumference would approximate the average circumference of the 

 head. Dr. Schultz, of this laboratory, in his anthropological study of the fetal 

 period, is utilizing the horizontal, transverse, and sagittal circumferences, the lat- 

 ter two being composed respectively of the transverse arc and biauricular diameter, 

 and the sagittal arc and nasion-inion diameter. By dividing his results by 3 he 

 obtains a still more accurate mean head circumference. It is probable, however, 

 that the head modulus as used may be relied upon as giving an approximate index of 

 the normal head size and the essential proportions of the latter to body length. 



The head-modulus data for the individual specimens were entered as dots on 

 charts 3 and 5, showing their correlation to the mean sitting height and weight. 

 The dots do not form as compact a cluster as those for sitting height in the cor- 

 responding charts 2 and 4; there is, however, a consistent pathway through which 

 a smooth curve could be drawn, showing the mean head modulus. The readings 

 taken from this curve are entered in table 5. Up to the thirty-second week the 

 plotted field could be definitely outlined and its upper and lower limits for the suc- 

 cessive weeks were entered in the same table, the few widely outlying entries being- 

 omitted. Beyond the thirty-second week there were enough data to complete a 

 mean curve up to term, but not enough to warrant a delimitation of the range of 

 variation. In table 5 were also entered the weekly increments in the mean head 

 modulus and the proportion of the head modulus to the sitting height, expressed 

 in percentages. 



Table 5. — Iliad modulus, representing head size in millimeters, based on the mean between greatest horizontal circum- 

 ference and the biauricular transverse arc. 



