EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 33 
one of “ protochordal plate.” Also in one of Assheton’s 
papers (796, Pl. 20, figs. 17 and 18) the author clearly figures 
the proliferating region in the entoderm here referred to. 
b. The Annular Zone of Proliferation.—How the 
hind end of this entodermal protochordal plate comes to 
fuse with the front portion of a median ectodermal down- 
growth of the ectodermal shield I have described for Tarsius 
in a former paper (1902). It will again be discussed further 
down. It is, however, necessary first to establish a fact 
already formerly insisted upon both by Bonnet (’84) and 
myself (790), viz., that when once the protochordal plate has 
made its appearance as a median, mesenchyme producing spot 
in the entoderm, the same mesenchyme producing properties 
become evident in peripheral regions of the entoderm. 
These regions have been named by Bonnet for the sheep 
the ‘* Mesoblasthof”’; shortly afterwards I have described 
them (790) for the shrew as an elongated ringshaped 
zone of entoderm which is situated under and somewhat 
outside the border of the ectodermal shield (Fig. 60), and 
which, slanting backwards from the protochordal plate both 
right and left, meets under the hinder part of the shield 
in the region where the mesoblast has acquired that median 
thickening which is known as the primitive streak, continued 
in the Primates into the connective stalk (Haftstiel). 
The presence of such an annular zone of mesenchyme 
producing entoderm has been very emphatically denied by 
such embryologists as Rabl, Keibel, and others, and in 
O. Hertwig’s latest manual he makes no mention whatever 
of it in the chapter on the ‘‘ Lehre von den Keimblattern.”’ 
This is all the more to be wondered at, because we shall see 
that also in lower vertebrates a similar participation of the 
entoderm towards mesenchyme formation can as little be 
denied. It seems to me that the energy with which these 
facts are ignored must have its origin in the strength of 
certain theoretical considerations with which a multiple 
origin of mesoderm! would clash. 
1 For myself, I have on another occasion (02, p. 84) expressed my sym- 
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