DEVELOPMENT OF THE EXTERNAL FORM OF THE BODV. 125 



The head attains its maximum relative size during the second month when 

 its weight is about 45 per cent, of the total weight of the embryo. There- 

 after it gradually diminishes in relative size until birth when it constitutes 

 about 26 per cent, of the total weight. The trunk is relatively largest during 

 the first month when its weight is about 65 per cent, of the total and then de- 

 creases to between 40 and 45 per cent, in the later fcetal months. The 

 extremities gradually increase in relative size after their appearance; the 

 upper extremities forming about 10 per cent, of the total weight at birth and 

 the lower about 20 per cent. 



Although the relative growth rates of the various organs have also been 

 estimated it does not seem desirable to include them in the chapter which 

 treats of the external form c Two organs might be mentioned, however, 

 which have a marked influence on the contour of the body ; that is, the heart 

 and liver. In embryos about 10 mm. long the heart constitutes nearly 4 

 per cent, of the total body volume. Thereafter it diminishes in relative size 

 to less than i per cent, at birth. Figures 8 5 and 86 show the marked protrusion 

 produced by this organ on the ventral side of the body. In embryos of 

 around 30 mm. the liver forms more than 10 per cent, of the total volume of 

 the body; in fact it is relatively large during both the second and third 

 months. Figures 87 and 88 show the effect of the organ on the contour of 

 the body, in conjunction with the heart. 



References for Further Study. 



BRYCE, T. H. and TEACHER, J. H.: Early Development and Imbedding of the Human 

 Ovum. Glasgow, 1908. 



His, W.: Anatomic menschlicher Embryonen. With atlas, 1880-1885. 



JACKSON, C. M.: On the Prenatal Growth of the Human Body and the Relative 

 Growth of the various Organs and Parts. Am. Jour, of Anat., Vol. 9, 1909. 



KEIBEL. F.: Die Entwickelung der ausseren Korperform der Wirbeltierembryonen, 

 inbesondere der menschlichen Embryonen aus der ersten 2 Monaten. In Band I, Teil 

 II of Hertwig's Handbuch der vergl. und experiment. Entwickelungslehre der Wirbeltiere, 

 1902. 



KEIBEL, F. and ELZE, C.: Normentafel der Entwickelungsgeschichte des Menschen. 

 Jena, 1908. 



KEIBEL, F. and MALL, F. P.: Manual of Human Embryology. Chap. VIII, 1910. 



KOLLMAN, J.: Handatlas der Entwickelungsgeschichte des Menschen. Jena, 1907. 



MALL, F. P.: On the Age of Human Embryos. Am. Jour, of Anat., Vol. 23, 1918. 



RABL, C.: Die Entstehung des Gesichtes. Leipzig, 1902. 



TRIEPEL, H.: Alter menschlicher Embryonen und Ovulationstermin. Anat. An- 

 zeiger, Bd. 48, 1915. 



