150 DEVELOPMENT OF ARTERIES IN FORELIMB OF PIG. 



which is supplied through a large aortic branch. This stem pushes through the 

 brachial plexus on its way to the limb mass and divides therein into two branches 

 which again join together. Out of the axial net arises a capillary net which carries 

 the blood to the marginal vein. 



The succeeding stage (8 mm.) shows, in relation to the nervous system, an 

 essential continuation of the former stage. 



The 11.7-mm. embryo shows a recognizable skeletal primordium. Humerus, 

 radius, and ulna are separable. In the brachial plexus the chief paths are dif- 

 ferentiated. The formation of the deep veins has begun. The limb arteries 

 enter on the medial side of the plexus and here divide. A branch breaks through 

 the ventral nerves and out of the plexiform mass which this branch forms the artery 

 of the limb develops, running dorsal to the median nerve. Many islands in its 

 course show its formation out of a "net" arrangement. While on one side the 

 breaking through of the plexus is simple, on the other, the stem, in breaking through, 

 splits into three branches, which, lying in the angle of the nerves, fuse together. 

 Muller remarks particularly that the arteries in the region of the root of the ventral 

 nerve-plate form an actual network of vessels which are characterized by their 

 particular relation to the nerve primordium. 



From the stem artery, after its exit from the plexus, there arises a dorsal 

 vessel-formation out of which develop the subscapular, posterior circumflex, and 

 profunda, probably also part of the interossea dorsalis, and recurrent radial. 

 Furthermore, the primordia of the median, radial, and ulnar, in the form of a net, 

 are recognizable, while the immediate lengthening of the stem artery forms the 

 volar interosseous. Superficially situated vessels form the primordia of the 

 superficial antibrachii. The remaining embryos (14 to 20 mm.) show the median 

 artery as the chief stem; all the chief branches are identifiable and the network of 

 the first primordium has dwindled. 



In his later comparative work Mliller (1904) dilates upon the importance of 

 this axillary net. He sees in it many segmental lateral branches of the aorta, which 

 break through the brachial plexus and are joined to the plexus, medially and later- 

 ally, by a longitudinal anastomosis. Only one of these remains in conjunction 

 with the aorta and forms accordingly the subclavian. The others between the 

 aorta and the longitudinal anastomosis disappear. In the youngest stage (5 mm.) 

 Mliller has been unable to find a plexus arteriosus, but quite certainly traces a 

 branch of the aorta into the capillary net. (This gap is filled by my 4.5-mm. pig 

 embryo.) Muller remarks that no one has yet been able to demonstrate in the 

 mammals a multiple segmental supply for the limb, as in the lower vertebrates. 

 He believes rather that the original vessel is single and subsequently divides, 

 assuming in its divisions a segmental arrangement. In his work in 1908 he is less 

 certain, owing to the appearance meanwhile of the work of Rabl (1906). The latter 

 demonstrated the multiple segmental arrangement in the penguin. Muller (1908) 

 investigated more thoroughly the sections in which Keibel came across two sub- 

 clavian when preparing his book. As a result he reinterprets his plexus axillaris 

 as being probably formed out of several segmental contributions. 



