EAELY ONTOGENETIC PHENOMENA IN MAMMALS. 245 
wall of the embryo, and as long as it is distinguishable from 
the protogenetic inesoblast (Hubrecht’s protochordal plate 
and annular zone mesenchyme) its anterior margin forms a 
landmark between the protogenetic and deuterogenetic areas. 
I have on a later page referred to a difficulty with 
reference to the notochord. But I will here ask Hubrecht 
two questions : 
Is any part of the notochord formed — to use his own 
terminology — by kephalogenesis ? That is to say, is there 
any notochord anterior to his supposed vermactinian stomo- 
daeum coalescence ? If he answers No, then I will ask how 
he explains his earlier observation, e. g. on Sorex, where the 
protochordal plate is shown to give rise to the anterior- 
portion of the notochord. If he airswei-s Yes, then I will ask 
how can it be due to coalescence of the vermactinian 
stomodmum ? 
Chapter II. 
So far as my own personal observations go I can support 
Hubrecht’s contention as to the origin of mesoblast (mesen- 
chyme) from hypoblast in front of the primitive streak, both 
as regards that in connection with the protochordal plate and 
that from the annular zone in the sheep and rabbit. 
Hubrecht describes the peripheral mesenchyme-producing 
region in these words: “ As an elongated ring-shaped zone of 
entoderm which is situated under and somewhat outside the 
border of the ectodermal shield and which, standing back- 
wards from the protochordal plate both right and left, meets 
under the hinder j3art 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 (Uaftstiel).” 
From my point of view this protochordal mesenchyme and 
annular zone mesenchyme which reaches round posterior 
and ventral to the primitive streak mesoblast is the mesen- 
chyme of protogenetic origin, while the primitive streak 
