146 



DEVELOPMENT OF ARTERIES IN FORELIMB OF PIG. 



margin than at the cranial (radial) end. The 

 caudal end shows a great venous plexus which 

 receives the ulnar vein (vena basilica) and, in 

 addition, veins that drain the dorsal surface of 

 the limb-bud. The vena cephalica is much more 

 indefinite than the vena basilica. 



Embryo 12 mm. (Plate 2, fig. 4). 



An embryo of this size discloses changes 

 which can be made out to some extent in one of 

 10 nun. This refers particularly to the branches 

 of the subclavian in the thoracic and cervical 

 regions. 



The limb-bud as a whole is occupied by a 

 central axial stem bounded by a marginal vein 

 and capillaries uniting the axial trunk to the 

 margin. The axial trunk is well defined as 

 far as the body-wall. Thereafter it becomes 

 retiform and continues in this condition until 

 it divides into a dorsal and a ventral system of 

 capillaries. The proximal portion represents 

 the brachial artery, the distal portion the volar 

 interosseous. In the carpal region there passes 

 dorsally a retiform mass of vessels which repre- 

 sents the ramus carpi dorsalis. Everywhere 

 the retiform arrangement becomes reduced to 

 capillaries which eventually reach the marginal 

 venous system. 



The subclavian has so increased in size that 

 the dorsal segmental artery at its origin has 

 been rendered inconspicuous. The vertebral can 

 be picked up as slender capillaries joining the 

 fifth and sixth and the sixth and seventh seg- 

 mentals. A little more distal a mass of capil- 

 laries have coalesced and become defined as the 

 posterior cervical of the swine. This represents 

 the thyroid axis of human anatomy. On the 

 caudal side of the subclavian the next three 

 segmental arteries have been joined together by 

 a capillary anastomosis. This represents the 

 dorsal artery of the pig and corresponds to the 

 superior intercostal and profunda cervicis of 

 human anatomy. As this artery becomes more 

 defined and larger, it will appear to supply the 

 first three intercostal spaces and its dorsal 

 branches will become distributed to muscles 

 of the back and neck. The first three inter- 

 costal, when traced laterally, are found to be 

 united by a capillary anastomosis and this 

 anastomosis establishes the internal mammary 

 artery. The marginal venous channel is com- 

 plete; along the cranial or radial aspect it forms 

 the vena cephalica, while along the caudal (ulnar) 

 margin it forms the vena basilica. The latter 

 is much the larger vein and reaches the cardinal 

 vein. Before its termination it bends cranially 

 and lies ventral and a little caudal to the sub- 

 clavian artery, thereby becoming the subclavian 



vein. Veins which accompany the internal 

 mammary open into it (the thoraco-epigastric 

 vein). Slender veins from the limb-bud also 

 reach it that is, veins which are about now 

 beginning to accompany the central artery of 

 the limb. 



Embryo 19 mm. (Plate 2, fig. 5). 



Before describing this stage, brief reference 

 may be made to stages intermediate between this 

 and the earlier stages that have been studied 

 but not figured. A series of transverse sections 

 of a 13-mm. embryo were examined. These 

 show the termination of the axial artery in a 

 dorsal and ventral capillary plexus, which ramify 

 between the differentiating musculature. A 15- 

 mm. injected embryo was studied, but this shows 

 no great advance over the stage last figured. 

 Transverse serial sections show the relation of 

 the main axial trunk of the limb-bud to the post- 

 cardinal vein and to the elements of the brachial 

 plexus. The main axial trunk takes up a posi- 

 tion between the dorsal and ventral elements of 

 the plexus. The points of origin of the post- 

 cervical and the dorsal artery are indicated, 

 as also the subscapular, and the plexiform 

 termination of the axial trunk is seen to ad- 

 vantage. An embryo of 16 mm. may also be 

 alluded to. The cervical and thoracic branches 

 of the subclavian are more defined but have not 

 yet emerged from a retiform condition. Axil- 

 lary and brachial branches of the axial artery are 

 beginning to coalesce and enlarge and the sub- 

 scapularis and circumflex can be identified as 

 plexiform groups. The radial and ulnar are 

 still undifferentiated from the capillary plexus. 

 The volar interosseous, which earlier was an 

 indefinite plexiform aggregation, has now become 

 a definite vessel which continues to the extremity 

 of the bud. The ramus carpi dorsalis is defined 

 and ends in four dorsal arteries, while the ventral 

 portion of the volar interosseous, which is more 

 slender, ends by joining with the digital branches 

 of the median artery, which can now be identi- 

 fied. It arises from the brachial, approaches 

 the ventral surface of the limb abruptly, and, 

 from being plexiform at its origin, soon becomes 

 a mass of capillaries which extend over the volar 

 surface to the margin of the limb. Included in 

 this capillary network are the radial and ulnar. 



Figure 5 shows an injected specimen of about 

 19 mm. length. The arterial side of the circu- 

 lation has now achieved, except in the extremity 

 of the limb, a definite tubular arrangement. 

 The veins, on the other hand, retain much more 

 of the primitive anastomosing arrangement. 

 It was found that by reducing the transparency 

 of the cleared specimen, the form of the skeleton 



