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



that it was stretched unusually straight, and in part to the fact that it shows no 

 formalin distention, owing to the moderate maceration of its tissues. Fetus B 

 would fall in closely with our mean curve, fetus A well below the curve, and fetus L 

 considerably above the curve. 



If it were not for the effect of the preservative, the correlation field would be 

 even more compact than it is. In other words, there is probably less variation 

 between the weight and length than is indicated in our charts. 



Rome fetuses of over 400 grams weight were embalmed with a 10 per cent 

 formalin solution injected through the umbilical artery. These are indicated in 

 the tables. Here the artificial increase in weight is considerable and these speci- 

 mens can not be fairly compared with those simply immersed in formalin. In 

 such cases, therefore, the fresh weight was used, plus 5 per cent as the equivalent 

 of the average increase due to formalin immersion based on the experiments of 

 Schultz (1919). The size recorded in these cases is that of the fresh specimen. 



MEASUREMENT. 

 Three measurements were selected for the purpose of correlation with weight. 

 The sitting height or crown-rump length (Mall) was taken as of primary importance, 

 inasmuch as it can be satisfactorily made from the youngest stages to term. It 

 possesses additional advantages in that in fetuses it can be determined more accu- 

 rately than the standing height and that it eliminates the individual variations in 

 the length of the lower extremities. In larger specimens the only precautions 

 found to be necessary were to hold the body straight and, in those of the last three 

 months of pregnancy, to keep the posture of the head uniform, which was done by 

 placing the head so that the eye-ear line was perpendicular to the axis of the body. 

 Where the fetus had been hardened in an extremely flexed position, so that an 

 accurate, straight measurement could not be obtained, the sitting height was not 

 charted; and where the curvature of the body was less marked the specimen was 

 entered' with an explanatory note in the tables. Such a specimen, instead of 

 measuring 148 mm., for instance, under normal conditions would probably measure 

 about 160 mm. and thus would tend to fall below the mean curve. On the other 

 hand, some are stretched unusually straight before fixation, with elongation of the 

 neck,' and under these circumstances they may measure as much as 5 per cent longer 

 than' usual, thus falling above the curve. All such specimens were noted in the 

 tables under "Remarks." Workers in other laboratories, utilizing the curves for 

 comparison with other material, should take these factors into account and make 



proper allowance. 



The body could be safely straightened for crown-rump measurement down to 

 the stage of 35 or 40 mm. long. Specimens smaller than this were measured without 

 disturbing their natural curved posture. These are indicated in the tables and in 

 the curve given on chart 1 they are entered by crosses instead of the dots used for 

 all straight measurements. 



For measuring the crown-rump length of small embryos, where it is desired to 

 obtain readings involving fractions of a millimeter, it is important to have some 

 device more accurate than small calipers. No matter how careful one might be, 



