Changes in the relative percentage weights of the organs in 187 fetal and 24 newborn dogs are presented. Nine organs increase less in total weight during fetal life than total body weight. These are, in increasing order: spinal cord, brain, suprarenals, testes, hypophysis, heart, thyroid, urinary bladder and liver. Increasing more than body weight and in increasing order are: ovaries, digestive system, uterus, spleen, lungs, thymus, digestive tube, kidneys and pancreas. No adult weights were found for seven of these, but the increases to adult weight are shown for the other organs.
The organs increasing less than body weight decrease in relative percentage weight rapidly at first and then more slowly throughout fetal life and all of these, except the heart and spinal cord, decrease in percentage weight in the adult.
Organs increasing more than body weight have their most marked increase in the early part of fetal life. Thus the relative organ weights are changing most rapidly in early fetal life with more gradual changes toward the end. The spleen and pancreas manifest the most marked and continued increase, while the cord, suprarenals, brain and testes decrease most precipitously in early fetal life. The kidneys, lungs and ovaries have their maximum percentages at birth.
These relative changes are compared with similar relative changes in man and in the cat.