EARLY ONTOGENETIC PHENOMENA IN MAMMALS, oO | 
the allantois based on facts that are not only furnished by 
the higher but also by the lower vertebrates. 
In studying the probiem how the vascularisation of this 
early raphe or connective stalk of mesoblast is brought about 
we must bear in mind that in a former chapter (p. 33), we 
have established that the starting-point for the vascular 
system, as we find it outside and inside of the embryo, is an 
annular zone of entoderm, which, at an early stage of the 
development of Tarsius, lays the foundation both of the 
blood-vessels and of the blood. 
We saw the endothelium of the heart derive from the 
entoderm cells in the anterior portion of the protochordal 
plate (02, Fig. 73, a, b), we saw the blood-vessels on the 
entodermal wall both in the intra- and in the extra-embryonic 
vascular regions take their origin out of the entoderm (02, 
Fig. 59, c—f), as this was also observed for Petromyzon by 
Goette (88,90) ; for Selachians, by Swaen and Riickert ; for 
Teleostei, by Swaen et Brachet (’99); for Amphibia, by 
Goette (75) and Brachet (’02, 703); for birds, by Balfour 
and Deighton; for mammals (sheep, Tupaja, Sorex), by 
Bonnet (?84, 89) and myself (90). 
Moreover, the most intense manifestations of the produc- 
tion of blood-vessels in Tarsius is given in the hinder region, 
where the annular zone of mesenchyme-producing eutoderm 
underlies the median zone of the ventral mesoblast (’02, 
Fig. 59, g—k). In earlier stages it is this posterior median 
zone (02, Fig. 54, g—k) which commences vascularisation of 
the mesodermal raphe we are here discussing, thus laying 
the foundation of the blood-vessels in the ‘ connective 
stalk”’ Now in this region of the stalk we can imagine the 
vasifactive phenomenon to have become exceptionally active 
if we remember that vascularising the ‘“ stalk ” means at the 
same time the possibility of direct vascularisation of the diplo- 
trophoblast. his direct vascularisation would undoubtedly 
constitute so great an advantage to those mammals possessed 
of it (see pp. 34 and 102) that we can well imagine a vascu- 
lar hypertrophy arising. Also in somewhat later stages 
