THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



233 



Arjt. ca.rJ. 



a new connection is formed with the axillary, while the original connection 



persists as the j ugulocephalic (Fig. 205). 



In a rabbit embryo of ten and one-half days a vein follows the border of 

 the lower extremity all the way round, connecting 

 on the cranial side with the umbilical and on the 

 caudal side with the posterior cardinal. This is the 

 primitive fibular vein, and from its course is homol- 

 ogous with the primitive ulnar vein of the upper 

 extremity (Fig. 203). From this time on, how- 

 ever, the course of development in the lower ex- 

 tremity differs from that in the upper. The con- 

 nection of the fibular vein with the umbilical is 

 soon lost. In older embryos (fifteen days) two 

 branches of the fibular vein have appeared; one 

 of these, the anterior tibial vein, begins on the 

 embryo of 14 days (n mm.), dorsum of the foot and extends diagonally proxi- 



Modified from Lewis. ,, . ,v * i 



mally, to open into the fibular in the caudal 



border; the other, the so-called connecting branch, begins as twigs in the ab- 

 dominal wall and tibial side of the extremity and opens into the fibular just 

 proximal to the opening of the anterior tibial (Fig. 204). Later the distal 



Bracljiit veiq 

 Ext. njArqyarj 



FIG. 204. FIG. 205. 



FIG. 204. Diagram of the veins in the extremities of a rabbit embryo of 14 days 



and 18 hours (14.5 mm.). Modified from Lewis. 



FIG. 205. Diagram of the veins in the extremities of a rabbit embryo of 17 days 

 (21 mm.). Modified from Lewis. 



part of the primitive fibular is broken up by the differentiation of the digits 

 (toes) and disappears almost up to the point of junction with the anterior 

 tibial. The latter enlarges and receives the digital branches, and appears 



