DEVELOPMENT OF ARTEKIES IN FORELIMB OF PIG. 



145 



DESCRIPTION OF REPRESENTATIVE SPECIMENS. 



Embryo 4.5 mm. (Plate I, fig. I). 



In this embryo the forelimb-bud shows as a 

 blunt elevation, appearing opposite the fifth, 

 sixth, seventh, eighth, and ninth segmental 

 arteries. The hindlimb is not apparent. From 

 each of these dorsal segmental arteries a lateral 

 branch arises. At its origin the lateral branch 

 is plexiform and its connection with the dorsal 

 segmental is multiple. These lateral branches 

 traverse the body-wall dorsal to the cardinal 

 vein and reach the limb-bud. In the forelimb 

 they become converted into a capillary network 

 which occupies the whole of the bud except a 

 clear marginal area. The vascular drainage 

 of the bud takes place by many veins which 

 open at irregular intervals into the cardinal vein. 

 At the cranial and caudal extremities the venous 

 tributaries extend into the body of the embryo 

 beyond the actual area giving origin to the 

 limb-bud. 



Macalister (1886), on theoretical grounds, 

 suggested that, as the limbs arise by the con- 

 solidation of the ventrolateral appendages de- 

 rived from several segments, each limb primarily 

 receives vessels from several metameric trunks. 

 Subsequent workers, however, succeeded only 

 in reducing the blood supply to the limb to a 

 single axial trunk. Miiller, from his compara- 

 tive studies, became convinced that the original 

 blood supply to the limb was in the form of a 

 capillary net and that this net was based on 

 polysegmental contributions. Evans and Rabl 

 showed that in bird embryos such a polyseg- 

 mental arrangement was the case. In 1910 

 Goppert showed the same for the mouse. Evans, 

 in his studies on the forelimb of the duck, 

 figured a still earlier stage in which the arteries 

 to the limb were not dominated by a metameric 

 arrangement. Also, Goppert showed in the 

 mouse an arrangement of blood-vessels to the 

 limb which, in the earliest stages, bore varying 

 relations to the cardinal veins and were not 

 altogether in metameric order. 



The embryos of the stage here described 

 (fig. 1) show none of the variability so much 

 stressed by Goppert and in all of the cases 

 studied the vessels are segmentally arranged, 

 appearing as lateral branches of the dorsal seg- 

 mentals. This polysegmental arrangement has 

 now been proved to hold for all the vertebrates 

 except amphibia. Of the mammals, the mouse 

 and the pig can now be included in the list and 

 there can be no reasonable doubt that the same 

 obtains for man. The presence of double sub- 

 clavise in the human embryo has been described 

 several times. 



Embryo 7.5 mm. (Plate I, fig. 2). 



This specimen shows the dominance of the 

 lateral branch of the seventh segmental artery 

 so enlarged that it constitutes the main axial 

 trunk of the forelimb bud. As it passes into the 

 bud it becomes coarsely plexiform and assumes 

 a retiform character. The components of the 

 rete diverge in a cranial and caudal direction and 

 in turn become broken up into dorsal and ventral 

 capillary networks. 



The contributions from the other segmentals 

 are disappearing. The remnants of those from 

 the fifth and sixth can still be observed, together 

 with a slight anastomosis between these branches. 

 Also a slender contribution from the eighth can 

 be picked out, but the contribution from the 

 ninth seems to have entirely disappeared. 



Venous communications exist on the dorsal 

 aspect, draining into the veins accompanying 

 the dorsal segmental arteries. Along the ventral 

 surface venous communications are established 

 with the lateral body-wall. At the cranial end 

 of the limb-bud venous communications are 

 established with the cardinal. A similar process 

 takes place at the caudal end. The periphery 

 of the limb is occupied by a capillary network 

 which will subsequently be changed into a con- 

 tinuous venous marginal channel. 



Embryo 8.5 mm. (Plate I, fig. 3). 



This stage shows the more definite arrange- 

 ment foreshadowed in the previous embryo. 

 The limb-bud is occupied by a dense capillary 

 network fed by a single axial trunk derived from 

 the seventh segmental artery. The axial trunk, 

 when followed into the limb-bud, assumes first 

 a retiform arrangement, ending finally in a capil- 

 lary net which diverges dorsally and ventrally. 

 Comparing this with the previous stages, we can 

 follow the gradual changes towards the forma- 

 tion of a definite axial vessel. First of all, there 

 is a diffuse capillary net, a sort of equi-potential 

 system in which each unit has the same dimen- 

 sional value. This is succeeded by a coarsely 

 plexiform arrangement a retiform stage in 

 which the elements are larger but still branching 

 and diffusely anastomosing. By coalescence, 

 out of this stage a definite stem will form. 

 These successive stages provide abundant op- 

 portunity for variation and the production of 

 anomalies. These embryos also provide evi- 

 dence for the theory of vascular formation, long 

 ago set forth by Baader (1866). 



The marginal vein is almost complete, but 

 along the tip of the bud it still retains its capillary 

 arrangement. The formation of definite veins 

 is much more advanced along the caudal (ulnar) 



