AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



55 



which the fourth arch had been very recently completed, and also of 4 at the 

 beginning of the post-branchial period, when the arches were about to lose their 

 identity. The measurements were divided by the magnification of the model 

 and corrected approximately for shrinkage. Between the two periods the average 

 length of each showed a negligible increase of less than 5 per cent. The failure 



Figs. 1 to 16. Ventral views of aortic-arch system, showing successive developmental stages. In the earliest stage 

 only the first arch is present, while in the last (a full-term fetus) the vessels have acquired nearly their 

 adult form. The so-called fifth arch is indicated by asterisk. Figure 1, embryo No. 2053, length 3 

 mm.; figure 2, embryo No. 12016, length 3 mm.; figure 3, embryo No. 836, length 4 mm.; figure 4, 

 embryo No. 588, length 4 mm.; figure 5, embryo No. 1075, length 6 mm.; figure 6, embryo No. 1380, 

 length 6 mm.; figure 7, embryo No. 810, length 5 mm.; figure 8, embryo No. 617, length 7 mm.; 

 figure 9, embryo No. 792, length 8 mm.; figure 10, embryo No. 1121, length 11 mm.;,figure 12, embryo 

 No. 1771, length 13 mm.; figure 13, embryo No. 940, length 14 mm.; figure 14, embryo No. 74, length 

 16 mm.; figure 15, embryo No. 1390, length 18 mm.; figure 16, full-term fetus. 



