EARLY ONTOGENETIC PHENOMENA IN MAMMALS. Oo” 
that for example the fact that in Tarsius after a certain 
time we find the whole umbilical vesicle thickly covered with 
blood-vessels (Hubrecht, ’02, Fig. 91) does not imply that 
they have arisen in loco out of the entoderm. ‘They have 
become spread over this after they had once taken their 
origin in the annular zone here more particularly alluded to. 
I think it may here suffice to give the reference to 
Mehnert’s article in which he establishes the entodermal 
origin of a ring of vasifactive tissue both for a reptile and 
for a bird, and not in this place to describe the process more 
in full. The more so as it is well known how many differences 
of opinion yet exist on this head between different authors. 
The amount of difference can also be gathered from Mehnert’s 
paper, who gives a tabular summary of the different opinions 
held on this point by no less than thirty-six different authors, 
grouped under the heads of six different possibilities for the 
origin of blood and blood-vessels. 
Having so wide a divergence to choose between, it is only 
natural that I should feel inclined to side with Mehnert 
(96), O. Hertwig (83, p. 319), Goette (74, ’75), His (’00), 
and Rickert (’06) im respect to the origin of the vascular 
system now that different genera of mammals have pro- 
vided me with perfectly trustworthy sections from which 
to conclude to the existence of the annular mesenchyme- 
producing zone of the entoderm. ‘That for mammals, 
Kolliker, Keibel, Heape, and others have denied the partici- 
pation of the entoderm here advocated, and have derived the 
whole vascular system from the mesoblast of the primitive 
streak has no doubt its explanation in this fact that they 
must have consulted later stages of development than those 
in which the entodermic origin is evident. This latter stage 
is very soon followed by one in which the participation of the 
entoderm has come to a close, and in which the further deve- 
lopment of the vascular system is now going on between the 
two primary layers in the so-called mesoblast. ; 
All the points here discussed have been sifted and care- 
fully compared by Riickert and Mollier in the chapter which 
