EARLY ONTOGENETIC PHENOMENA IN MAMMALS, 29 
structures is budded off only from the primitive streak, and 
who—some of them at least—even wish to derive the vascular 
system and the blood from the same source. Mesenchyme 
formation, so sharply distinguished by O. Hertwig from 
mesoblast formation (see his ‘ Lehrbuch,’ ed. 1906, p. 218) 
is by many authors held to be of no significance whatever in 
mammals, although Bonnet, in his investigations on the 
sheep’s development (82, ’89), has attempted to stem that 
current of thought in demonstrating for the sheep that: the 
vascular region on the yolk-sac is a direct derivate of local 
proliferation of the entoderm. In his later publications on 
the dog, however, Bonnet has for that mammal denied the 
presence of a similar process, although from his plates (01, 
Pls. XVIII, XIX; fig. 6, and many others) another conclu- 
sion might certainly be drawn (Figs. 91 and 92). On the 
contrary for Sorex and Tupaja (as yet unpublished) the 
genesis of mesenchyme out of entoderm has been fully con- 
firmed by myself, and the region in which the participation 
of the entoderm towards the formation of blood-vessels and 
blood occurs, has been figured in detail by me (790, Figs. 
58, 61). When seen from above the aspect is such as to 
warrant the designation of this region by the name of the 
annular zone of mesoblast-producing entoderm of the shrew 
and of Tupaja. 
Since then the battle has been raging concerning this very 
difficult and yet very important question of comparative 
embryology round which many problems, connected with the 
interpretation of the germinal layers and the significance of 
mesoblast, centre. 
Only very lately Riickert has given a remarkable digest— 
in co-operation with Mollier—in Hertwig’s ‘ Handbuch,’ Vol. 
I, p. 1244—1260, in which—starting from careful investiga- 
tions—he draws important conclusions concerning blood- 
formation in all the Vertebrates, that go far to demolish part 
of the theoretical views held by Rabl on mesoblast formation, 
which latter have been largely accepted by the great majority 
of embryologists. 
