452 ON INCREASE IN SIZE [pt. m 



From this it can be seen that the organs which reach their maximum 

 relative weight early in development are the heart, spleen, pan- 

 creas, thymus, brain, spinal cord and head. The mesonephros also, 

 of course, reaches its maximum fairly soon and declines more or 

 less rapidly afterwards. The muscle masses, shown especially in the 

 figures for fore and hind limbs, increase steadily in relative weight 

 and reach their maximal relative size at or shortly before birth. The 

 suprarenal gland, the stomach, the lungs, and the thyroid are variable 

 in their point of maximum. But, as can be seen from the table, the 

 data on these matters are not very numerous, most of the attention 

 which has been given to the relative weights of parts and organs 

 having gone into the study of post-natal life. W. Schultze has made 

 an interesting investigation on the effect of hormones on the de- 

 veloping parts and organs in the tadpole. 



It is interesting that the only conclusions to which Jackson would 

 commit himself were ( i ) that the embryo grows much faster in the 

 earlier stages than in the later, and (2) that, at any rate as far as 

 vertebrates were concerned, pre-natal growth is relatively much 

 greater at the cephalic than at the caudal end. These points had 

 already both been stated by Aristotle, and the whole advance lay 

 in giving them a quantitative backing. Jackson did not consider that 

 his figures supported the view of Preyer that those organs grow fastest 

 in the embryo which will afterwards first come into functional 

 operation. 



A great mass of such data has since Jackson's time been collected 

 by Calkins & Scammon and many investigators working under their 

 influence. We heed not do more than mention their work on the 

 growth of the spinal axis in the human embryo, that of Scammon 

 on the height-weight index, Nafiagas on anencephalic embryos, 

 and Brody and Hammond on proportions in the cow, for these 

 and many others only indirectly concern us. But it is important to 

 note that for the human embryo Calkins & Scammon found that 

 from 3 months onwards the growth in length, girth and diameter 

 of the various external divisions of the body was directly proportional 

 to the growth in total body-length. While each dimension has its own 

 growth-rate with respect to the total body-length, this characteristic 

 rate does not alter during the period under consideration. All these 

 entities then may be expressed by the Calkins equation 



D =^ aL±b, 



