136 C. M. Jackson. 



(average of 26 full-term still-born) or .125 per cent (average of 11 

 born alive). Vierordt gives .16 per cent of the total body weight 

 for the thyroid gland in the new-born, decreasing to .05 per cent in 

 the adult. 



Thymus. 



In the youngest specimen measured (end of 2d month), the thy- 

 mus formed on^y .008 per cent of the entire body. As may be seen in 

 Tables IV and V, it is subject to extreme fluctuations in relative size, 

 being in this respect one of the most variable of all the organs. When 

 the average of a considerable number of specimens is taken, how- 

 ever, as shown by the curve in Fig. 4 (from data in Table V), the 

 increase in the relative size of the thymus is evident. The average 

 of all observations available gives .326 per cent of the total body for 

 the thymus in 124 full-term still-bom, and .313 per cent for 101 

 born alive. According to Vierordt, the thymus decreases from .26 

 per cent of total body weight in the newborn to .04 per cent in the 

 adult. 



Heart. 



In the early embryo, -the heart is relatively large. In the youngest 

 sjiecimen directly measured (5th week, 11 mm.), the heart formed 

 3.64 per cent of the total body volume. On the His-Ziegler model 

 of a 4 week embryo (A, T.5 mm.), I have taken measurements from 

 which the heart is estimated to form more than 5 per cent of the 

 total body volume (as indicated by the dotted line at the beginning 

 of the curve of relative growth for the heart in Fig. 3). From this 

 curve, and from the data in Tables IV and V, it is evident that the 

 heart decreases rapidly in relative size, dropping to .85 per cent in 

 the 3d month. It continues fairly uniform in relative size from 

 the 4th month onward, usually averaging between .7 per cent and .8 

 per cent for each month. In 165 full-term still-born the average 

 was .70 per cent ; and in 164 born alive it was .77 per cent. Ac- 

 cording to the estimate of Vierordt, the heart forms .76 per cent of 

 the total weight in the newborn, and .46 per cent in the adult. 



