CELL CLUSTERS IN MAMMALIAN EMBRYOS 



409 



vertebral column, but is an actual shifting of these ventral 

 branches when compared with the dorsal branches of the same 

 trunk (Evans, '12, p. 648). 



Although the details have not, so far as I am aware, been 

 as carefully ascertained as in the case of the human embryo, 

 essentially the same conditions evidently maintain for the pig 

 embryo as in other mammals. The ventral and lateral aortic 

 rami undergo a similar degeneration. The shifting of the 

 ventral or intestinal vessels is indicated by the fact that the 

 coeliac artery which in a 6.5 mm. pig embryo is at the level 

 of the eighth cervical segment, in a 12 mm. embryo is at the 

 level of the fifth thoracic segment. Again, the superior mesen- 

 teric artery in the 6.5 mm. embryo is at the level of the third 

 thoracic segment, but has descended to the level of the eighth 

 thoracic segment in the 12 mm. embryo. Both the atrophy 

 of vessels and caudal wandering appear practically complete 

 at about the 15 mm. stage. 



2. Correlation of the aortic clusters with these vascular changes. 

 In a comparative analysis of the preceding data for the vascular 

 changes and cell clusters in the aorta certain striking relation- 

 ships become evident. First, both phenomena occur within 

 the same period of embryonic development — between the stages 

 of about 5 mm. to 15 mm. in the pig embryo. Again, both 

 the formation of the clusters and the degenerative changes and 

 caudal wandering of the arteries, as already indicated, are con- 

 fined to the same region of the aorta, namely, its ventral wall. 

 Third, in a linear direction within this ventral region the cell 

 clusters are furthermore fairly evenly distributed between the 

 coeliac and umbilical arteries as shown in the following data 

 for four 12 mm. embryos: 



