SECT. 2] AND WEIGHT 383 



merit, but changes from very slightly above i-o in the early stages 

 to 1-05 in the later ones. Probably this is associated with the pro- 

 gressive loss of water as the embryo develops. 



One of the first to investigate quantitatively the growth of the 

 human embryo was Boyd in 1861, who studied the weights of all 

 the principal organs in embryos from 8*5 to 85 oz. He did not give 

 figures for individual embryos from which a curve could be con- 

 structed, but simply divided them into large groups such as "pre- 

 maturely-born", etc. Legou's data, already referred to, were worked 

 over again in 1903 by Loisel. Zangemeister, more recently, has 

 published figures for human embryonic growth — these are shown in 

 Table 17 of Appendix i. 



Other data for embryonic growth in man will be found in the papers 

 of Fesser ; Toldt ; Meyer ; Heuser ; Bedu ; Sombret ; Arnoljevic ; 

 Stratz; Borri; Corrado; Balthazard & Dervieux; Ecker; Hamy; 

 Kolliker; Cruickshank & Miller; Browne; and Friedenthal. Scam- 

 mon & Calkins, who have made a great many measurements in 

 recent years, have constructed a three-dimensional isometric pro- 

 jection, from which the height, weight and age of a human embryo 

 may be read off if any one of them is known (see Fig. 33). The best 

 recent paper on the whole subject is that of Streeter. 



Sandiford has shown that the weights and surface areas of foetuses 

 fall on straight lines when plotted on double log paper. For further 

 information on surface growth see Scammon & Klein. 



(j) Whale. Some information on the embryonic growth of the 

 whale is contained in the papers of Harmer; Risting; Hinton; and 

 Mcintosh & Wheeler, but it mostly concerns increase in length. 



2-3. The General Nature of Embryonic Growth 



We may now turn to the theoretical aspect of the matter in the 

 attempt to find out what interpretation can be placed upon them. 

 We may in the first place take as a simple example of an embryonic 

 growth-curve the work on the growth of the chick (White Leghorn) 

 of H. A. Murray. Table 51 shows, firstly, the actual weights of the 

 embryos on each successive day, secondly, the amount gained in each 

 such 24-hour period, i.e. the amount of substance actually added on 

 to itself by the embryo during the lapse of the time in question. This 

 is known as the daily increment. In the next column the averages 

 of the daily increments are placed, and these figures, known as the 



