376 A. A. W. HUBRECHY. 
this stage the mesoblast of the primitive streak, which lies 
partly beneath the epiblast of the germinal area, partly 
stretches beneath the trophoblast when we examine it further 
backwards than the point of insertion of “ germinal area 
epiblast ” against trophoblast. Now, if we suppose a similar 
state of things to be extant in the human embryo, and if at 
the same time we assume—what may safely be done know- 
ing the accelerated development which characterises the 
human embryo—that the dorsal arteries, parallel to the axis 
of the embryo, arise very early and attain considerable 
development simultaneously with the origin of the vitelline 
circulation, we can then easily understand that these arteries 
are at such an early stage prolonged into the primitive 
streak mesoblast, and thence further backward towards the 
somatic mesoblast which is applied against the trophoblast. 
If we now consider that already in Sauropsida (cf. Balfour, 
‘Comp. Embryology,’ vol. ii, p. 159, fig. 128@) it is this 
median primitive streak mescblast which takes an unmistak- 
able part in the formation of the mesoblastic tissue of the 
allantois, it is certainly no strained assumption to say that 
during this further passage backwards these arteries and their 
branches are only following the hereditary tendencies by which 
the formation of vascular allantoic mesoblast is determined. If 
now we suppose the separating of splanchnopleura and somato- 
pleura to take place it will be easily understood that this 
separation need not affect the region we are here discussing, 
because it is situated in the prolongation of the 
embryonic axis. 
And so all conditions are indeed fulfilled, by which the, per- 
manent connection (of which we have above traced the epi- 
blastic constituent) between embryo and trophoblast can also 
obtain a mesoblastic support along which at the same time the 
umbilical arteries travel towards the surface of the blastocyst, 
there to spread out rapidly and to form a network of vessels, 
which in the human embryo vascularise the mesoblastic layer 
of the blastocystic wall at such a very early period.! 
1 Judging from personal experience, J would venture to assert that the 
