152 DEVELOPMENT OF ARTERIES IN FORELIMB OF PIG. 



At stage 4 the limb-arteries still show multiple origin. They anastomose 

 on the medial side of the plexus, pass through the plexus, usually just cranial to 

 the third root. Before passing through the plexus a descending branch forms 

 the ramus caudalis medialis. After passing through the plexus a branch goes to 

 the radial margin of the limb, the ramus dorsalis. The main stem follows the 

 limb axis and breaks up into numerous branches and twigs. 



At stage 6 the arteries to the limb, in all except one specimen, are reduced to 

 a single trunk. Branches such as the volar interosseous, dorsal interosseous, and 

 internal mammary are recognizable. At stage 7 the main artery to the limb, in all 

 cases, springs as a single vessel and all its branches can be identified. 



In his discussion Goppert recognizes the stage we have called retiform as a 

 stage preceding the formation of definite stems, and he appeals to the postulates 

 of Thoma to explain the transformation. He has difficulty in accepting the 

 terms "plexus arteriosus subclavius," etc., of Miiller, because the individual ele- 

 ments of the plexus are much too large to be called capillaries. Our method dispels 

 this difficulty, as we show the capillary net preceding this retiform stage. 



SUMMARY. 



In the mammalian forelimb the earliest vascular pattern that we have suc- 

 ceeded in portraying is characterized by regular segmental contributions from the 

 fifth, sixth, seventh, eighth, and ninth segmental arteries. This contribution is 

 somewhat retiform at its origin and in the limb elevation becomes reduced to a 

 plexiform capillary net. Although this contribution happens to be segmental in 

 origin, yet in the limb-bud there is not the slightest trace of segmentation in the 

 vascular supply. In the duck, Evans (1909) discovered an arrangement of blood- 

 vessels to the forelimb which did not exhibit the regular metameric order. Goppert, 

 in the white mouse, similarly pictures a stage in which great irregularity and varia- 

 bility mark the earliest blood-supply to the limb. Perhaps these investigators 

 have succeeded in demonstrating an earlier phase than I have. In the formation 

 of an individual arterial tube three stages can be distinguished: (1) the stage of the 

 capillary net, which can be best elicited when the vessels are injected; (2) the stage 

 characterized by enlarged tubes showing island-formation, coalescence, and a 

 tendency to fuse the retiform stage; (3) the formation of the definite stem. 



These stages stand in phylogenetic order, the first being the most ancient; 

 also, they are repeated ontogenetically. Again, each stage is a response to definite 

 physiological demands, the first being an angioblastic response to tissue needs, the 

 second taking place according to the postulates of Thoma, and finally leading into 

 the third. 



Out of these available arteries of the forelimb the seventh soon dominates 

 and the others dwindle. It is hard to resist the inference that the seventh pre- 

 dominates because it is opposite the center of the growing limb-mass. For a time 

 we now have a growing limb-mass filled with a great capillary network and main- 

 tained by a central stem increasing continually in capacity. This holds until about 

 the 10-mm. stage, when the plexus on the medial side of the transparent area begins 

 to sort itself out into definite stems. 



