38o 



ON INCREASE IN SIZE 



[PT. Ill 



and Balthazard also gives figures for this, which will be found in 

 Appendix i (Table i6). The earlier workers, Ahlfeld; Fehling; 

 Hennig; Legou; Faucon; and Michaelis all obtained valuable data, 

 but it was not until 1909 that a critical examination of them was 

 made by Jackson who analysed the figures of his predecessors, and 

 added a large number of new ones. His results gave a continuous 

 curve from the earliest stages till birth, which agreed with the 

 majority of the other investigators, but not perhaps with Hennig's 

 curve (he gave no figures), 

 which showed a very distinct 

 slackening of growth about 

 the sixth month, after which 

 the same rate was resumed. 

 If this phenomenon is real, it 

 may possibly be associated, 

 as Donaldson has suggested, 

 with a transition from one 

 growth-cycle to another, at 

 the end of the sixth month, 

 when the absolute weight 

 begins to rise so rapidly. On 

 the other hand, the mass of 

 data which Quetelet and 

 others after him have ana- 

 lysed regarding the growth 

 of man throughout life, would seem to show that there are three 

 growth-cycles only, one pre-natal one, one with its maximum at 

 5*5 years, and the third with its maximum at 16 years. Vignes' 

 S-shaped curve for human embryonic growth is shown in Fig. 31. 



Bujard in 19 14 made a geometrical analysis of the early stages of 

 the human embryo. 



Jackson measured the volume and weight of all the specimens in 

 his own collection, and for the early stages also the volumes of the 

 His-Ziegler models. His figures are given in Tables 17 and 18 of 

 Appendix i, and the curve which he constructed from his own data 

 as well as those of previous investigators is reproduced in Fig. 32. 

 The 1 6th table of the appendix shows the volumes of the His- 

 Ziegler models, and demonstrates that the human embryo, like all 

 others, is much exceeded in size by the yolk-sac during the earliest 



3 4 5 

 Months 



Fig. 31- 



.270 290 310 



Days 



