84 H. D. SENIOR 
At the stage of 24.5 mm. (fig. 10, C) the direct arterial route 
upon the anterior surface of the m. popliteus has ceased to 
exist. The part of the a. poplitea profunda not incorporated in 
the definitive a. poplitea or tibialis anterior is represented: 
1) by the root and a small branch of the a. genu inferior medialis 
of the former and 2) by the a. recurrens tibialis posterior of the 
latter. The main arterial channel through the popliteal region 
has thus been transferred from the anterior to the posterior 
surface of the popliteus muscle. Both the a. poplitea and the 
a. tibialis anterior have assumed their adult arrangement. 
The formation of the adult a. poplitea results from the com- 
bination of two embryonic elements. The part of the vessel 
which extends from the hiatus tendineus to the origin of the a. 
genu inferior medialis is derived from the embryonic a. poplitea 
profunda. The remainder of the artery corresponds to the 
embryonic a poplitea superficialis. 
The embryonic components of the a. tibialis anterior consist 
of the r. communicans medius, a short section of the distal end 
of the a. poplitea profunda, the proximal part of the r. perforans 
cruris, and the entire tibialis anterior, pars distalis. 
The a. poplitea superficialis reaches completion at the stage 
of 24.5 mm. by extending as far as the r. communicans medius 
which then becomes one of its terminal branches. By this 
means the a. peronaea relinquishes its temporary participation 
in the formation of the a. tibialis anterior. 
B. The conversion of the embryonic a. tibialis posterior super- 
ficialis into the adult a. tibialis posterior. 
The a. tibialis posterior superficialis is originally a branch of 
the a. poplitea profunda and extends into the sole. The distal 
portion of the artery, marked off from the remainder of the 
vessel upon the development of the a. plantaris lateralis, becomes 
the a. plantaris medialis. The proximal portion of the a. tibialis 
posterior superficialis has blended with the corresponding section 
of the a. peronaea posterior superficialis at the stage of 24.5 mm. 
as far as the termination of the popliteal artery. The further 
blending between these two arteries which is to occur during 
the later stages of development will cause the migration of the 
