VASCULOGENESIS IN THE CAT 45 



are scattered but not very numerous anlages in the cardinal line, 

 and in the interval between the two. Many in the last position 

 are now coalescing with the umbilical vein, which is assuming 

 its function as a somatopleuric drainage channel. Caudally it 

 turns dorsad in the wall of the amnio-embryonic cavity and termi- 

 nates among vasofactive cell-masses. Conditions opposite the 

 first somites are shown in figure 16. The dilated cephalic ex- 

 tremity of the umbilical vein shows evidence of its origin by 

 coalescence in the presence of several perforations and septa. 

 At the level of the third somite it has effected a connection with 

 the omphalo-mesenteric vein which crosses it ventrad at right 

 angles. The intervening compact mesoderm lateral to the coe- 

 lom has given way over a small area (fig. 17) through which the 

 anastomosis between the veins is established. Near the cephalic 

 margin of the third somite a transverse vessel is forming and is 

 continued forward obliquely to the front of the first somite where 

 it is continuous with the channel along the neural tube. A com- 

 parison of these vessels with the angiocysts of figures 10 and 14 

 makes evident their correspondence in position. There is further 

 a small communication between the transverse vessel and a branch 

 of the aorta. Several small and as yet unannexed vesicles are 

 present. 



The whole length of the umbilical line in a twelve somite em- 

 bryo (Columbia Collection No. 547) is shown in figures 12 and 

 13. The somatopleuric mesenchyme is seen from below, the 

 mesoderm having been omitted as far mesad as the nephrotomes. 

 Cephalad the vein is represented by a large multilocular cyst 

 which extends from the limb bud, in which is a blood island, to 

 the level of the fifth somite. Then follow a series of angiocysts, 

 which are coalescing opposite the sixth and seventh somites but 

 farther caudad are scattered and often widely separated. Vaso- 

 factive cells amid which clefts have appeared are shown in yel- 

 low. There are numerous discrete vesicles in the cardinal line, 

 which being dorsal to the nephrotomes are not well seen in the 

 figure. Opposite the last somites the mesenchyme forms a net- 

 work of cords, the solidity of which is necessarily exaggerated 

 by the reconstruction. The resemblance to Bremer's angioblast 

 cords is striking. 



